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ISLAMIC MEDICAL EDUCATION RESOURCES-03

0509-ISSUES IN COMMUNICABLE DISEASES (PART I)

By Professor Omar Hasan Kasule Sr.

Learning Objectives:

Incidence and prevalence

Attack rates (primary & secondary), case fatality ratio.

The causal triangle: agent (types of micro-organisms, transmission routes with examples for each route), host (susceptibility), and disease (natural history, clinical features)

Methods of transmission

Difference between control and eradication of communicable disease

Prevention of communicable disease (primary, secondary, tertiary) and surveillance

Investigation and management of a disease out-break

Emerging and Re-emerging Diseases

 

Key Words and Terms:


Agent

Attack rate

Bacteria

Carrier state

Case fatality ratio

Communicable disease

Contagion

Cross-infection

Culture of micro-organisms

Disease reservoir

Disease control

Disease eradication

Disease outbreak

Disease vector

Drug resistance

Germ

Horizontal transmission

Host

Immunization

Incubation

Infection

Isolation

Latency

Microbial sensitivity

Micro-organism

Natural history

Parasite

Prevention: 1ry, 2ry, 3ry

Reservoir host

Sexually transmitted disease

Superinfection

Tachyphyllaxis

Trans-infection

Transmission

Vaccination

Vector

Vertical transmission

Virus


ISSUES IN COMMUNICABLE DISEASES

 

OUTLINE

EPIDEMIOLOGICAL CHARACTERIZATION

DISEASE

Definition of Communicable disease

Infection and Invasion

Pathogenicity (invasiveness, toxigenicity, and hypersensitivity)

Virulence

Disease: epidemic (disease outbreak), pandemic, endemic

 

EPIDEMIOLOGY

Communicable disease model/triangle (Triad 1: agent – host – disease OR Triad 2: agent – host –environment)

Rates & ratios: primary attack rate, secondary attack rate, and case fatality ratio

3 types of prevention: Primary prevention, Secondary prevention, and Tertiary prevention

 

Figure #1: DIARRHOEAL and RESPIRATORY INFECTIONS: INCIDENCE, and PREVALENCE by AGE and GENDER (per 100,000 population)

 

Organism: disease

Age group

Incidence-males

Incidence: females

Prevalence: males

Prevalence: females

Diarrhoeal Diseases

(Table 48)

0-4 years

5-14 years

15-44 years

45-59 years

60+

All ages

366, 868

57, 944

33, 439

30, 015

28, 596

78,098

354, 620

58, 212

35, 677

30, 927

27, 884

76, 578

6, 564

983

563

506

482

1,366

6, 594

987

601

521

470

1,365

Lower respiratory tract infections

(Table 105)

 

0-4 years

5-14 years

15-44 years

45-59 years

60+

All ages

41, 130

5, 574

941

915

2, 997

6, 936

41, 178

5, 576

940

913

2, 828

6, 824

1, 112

149

25

23

76

186

1, 114

149

25

23

71

163

Upper respiratory tract infections

(Table 107)

 

0-4 years

5-14 years

15-44 years

45-59 years

60+

All ages

536

359

316

288

378

363

536

360

320

305

381

358

5, 897

3, 954

3, 483

3, 172

4, 164

3, 893

5, 901

3, 964

3, 524

3, 358

4, 200

3, 944

 

Figure #2: DIARRHOEAL and RESPIRATORY INFECTIONS: INCIDENCE, PREVALENCE, AVERAGE AGE AT ONSET, and MORTALITY by REGION

Disease

Parameters

Established

Market

Economies

Former

Socialist

Economies

India

China

Other

Asian &

Pacific

Sub

Saharan

Africa

Latin

America & Caribbean

Middle

Eastern

Crescent

Diarrhoeal Diseases

(Table 48)

Incidence

Prevalence

Age at onset

Mortality

20, 963

366

18.9

0.3

27, 089

476

14.6

1.2

92, 747

1,632

9.8

108.5

89, 116

1,460

24.0

8.2

72,772

1,386

12.6

58.2

127,995

2,354

8.2

186.2

97, 804

1,707

11.6

34.5

85, 539

1,566

8.4

84.4

Lower respiratory tract infections

(Table 105)

 

Incidence

Prevalence

Age at onset

Mortality

1,160

22

35.3

34.1

4,472

86

13.1

32.6

8,606

236

9.6

141

6,502

178

8.6

41.2

7,857

215

7.7

79.6

10,796

296

7.0

197

7,813

214

7.4

40.1

9,428

258

6.5

104.4

Upper respiratory tract infections

(Table 107)

 

Incidence

Prevalence

Age at onset

Mortality

335,869

3,695

38.0

0.3

335,158

3,687

36.0

0.2

424,736

4,672

23.8

1.4

269,823

2,968

29.3

0.4

421,279

4,634

23.7

0.8

438,099

4,819

18.7

2.0

331,098

3,642

25.4

0.4

332,520

3,658

22.9

1.1

 

 

MICRO-ORGANISMS

CLASSIFICATION

Pro-karyotes, eukaryotes, and viruses;

feco-oral, soil contact, water contact, skin contact, air transmission, body fluids, arthropod-borne

Infectivity, virulence, pathogenicity, toxigenicity, and the infective dose.

Saprophytic, parasitic, commensals, and symbiotic, Obligate parasites.

 

DESCRIPTION

Microscopy, Serology, Gram Stain, Culture, biochemistry, DNA

Fungi less common

Protozoa: plasmodium, toxoplasma, and pneumocytes

Helminthes: schistosomiasis, hookworm,

 

TRANSMISSION

Reservoirs

Common vehicle spread

Serial transmission

exogenous infections vs endogenous infections.

Auto-infection

Horizontal transmission vs vertical transmission

 

PORTALS OF ENTRY

respiratory tract

urogenital tract

alimentary tract

mucous membranes

skin

placenta

parenteral portal

 

ROUTES OF TRANSMISSION

Air-borne infections

Vehicle borne transmission

Vectors

Biological transmission

Zoonoses & Anthroponooses

Plants

 

THE HOST

intermediate hosts

definitive hosts

reservoir hosts

accidental hosts

carrier status. healthy, incubational, convalescing, or chronic carriers

Susceptibility

Immune resistance

 

THE DISEASE

DEFINITIONS

severity: mild, moderate, severe, and fatal.

manifestations are: asymptomatic disease, latent disease, sub-clinical disease, and clinical disease.

natural history: pre-pathogenesis, pre-clinical, clinical, and chronic.

 

CONTROL and PREVENTION

Control of Outbreak: identification of cause, notification, treatment of cases using drugs, prevention, and surveillance.

Control Strategy: attacking the agent at source, interrupting transmission, and reducing the susceptible population.

Healthy host: active immunization, passive immunization, chemoprophylaxis, behavioral change (sexual, dietary), physical isolation, and increase of host resistance by better nutrition and health care.

Diseased host: chemotherapy, isolation, quarantine, restriction of activity, and behavioral change.

Vector control: chemicals, environmental and biological control.

Control of animal reservoirs: active immunization, restriction of movement or reduction in number, chemoprophylaxis and chemotherapy.

Environmental control: water sanitation, safe drinking water, excreta disposal, and food sanitation.

Control of the agent: cleanliness, refrigeration, disinfection, and sterilization.

Disease notification plays a central role in disease control.

 

FIGURE #3: COMPARING CONTROL AND ERADICATION

 

 

Control

Eradication

Aim

Minimal disease incidence

Complete elimination of disease

Length of program

Continuous

Limited duration

Area covered

Areas of highest incidence/preventive priority

Cover entire area to prevent recurrence

Methods

Effective

Faultless

Reservoir

Animal or environment

Human only

Organization

Good and routine

Perfect as special  temporary project

Costs

Moderate but over a long time

High for a limited time

Complications

Acceptable

Extremely serious

Imported cases

Not important

Very important

Surveillance

Reasonable

Very good

 

 

Figure #4: IMMUNISATION FOR INTERNATIONAL TRAVEL

Disease

Organism

Type of vaccine

Yellow fever

Live attenuated

 

Cholera

Heat-killed V. cholerae

 

Typhoid

Monovalent vaccine

 

Hepatis A

Live attenuated & Immunoglobulin

 

Malaria

Chemoprophyllaxis

 

 

 

Figure #5: IMMUNISATION SCHEDULE FOR CHILDREN – US

Age

Vaccination given

Birth

HBV 1

2 months

DTP 1, OPV 1, Hib 1, HBV 2

4 months

DTP 2, OPV 2, Hib 2,

6 months

DTP 3, OPV 3, Hib 3,

15 months

MMR 1, DTP 4, Hib 4

6-18 months

HBV 3

4-6 years

DTP 5, OPV 4, MMR 2

14-15 years

Td

 

DTP = diphtheria, tetanus, pertussis

HBV = heaptitis B conjugate vaccine

Hib = Hemophilus Influenzae type b conjugate vaccine

MMR = measles, mumps, and rubella

OPV = oral polio vaccine

Td = tetanus and diphtheria vaccine with a reduced amount of diphtheria

 

 

EMERGING AND RE-EMERGING DISEASES

DEFINITION

Causes: socio-demographic, lifestyle, human behavior, environmental, and medical technology

Old diseases and old problems.

Old diseases and new problems

New diseases and new problems

 

EMERGING AND RE-IMERGING DISEASES

Old STDs: syphilis, gonorrhoea, and chanchroid.

New STD: HIV, chlamydia, genital warts, trichomonas, scabies, peduculosis, genital herpes, vaginal candidiasis, E. histolytica infection, G. lamblia, HVA, HBV, HCV

New Viral: Ebola/Marburg, swine flu, and Lassa fever.

Old Viral: dengue fever, dengue hemorrhagic fever, Hepatitis B, hepatitis C, Rift valley fever and  Yellow fever.

New bacterial: toxic shock syndrome

Old bacterial: cholera, Escheria coli, Helicobacter Pylori, and tuberculosis

New protozoal: Lyme disease and Legionnaire’s disease

Old protozoal: Malaria and schistosomiasis

ISSUES IN COMMUNICABLE DISEASES

SYNOPSIS

 

1.0 EPIDEMIOLOGICAL CHARACTERIZATION

1.1 Communicable disease or infectious disease is disease transferred from person to person by micro-organisms. Infection is lodging, growing, and multiplication of micro-organism in the host’s body. Invasion is mere presence of micro-organism without necessarily multiplying. Infectivity is the ability of the agent to lodge and grow in the host. Pathogenicity is the ability of the agent to cause disease. Virulence is the ability to cause severe disease.

 

1.2 Pathogenicity is affected by invasiveness, toxigenicity, and hypersensitivity. Agents cause disease by invading tissues (eg shigella), producing toxins (eg botulinum), or causing allergic reactions (like Mycobacterium tuberculosis).

 

1.3 An epidemic is said to occur when the epidemic threshold is breached. The term epidemic is used to refer to a wide-spread disease whereas the term outbreak is used to refer to localized disease.

 

1.4 The communicable disease model comprises of the agent, the host, and the environment. Disease transmission occurs when a susceptible host and a pathogenic agent exist in an environment conducive to disease transmission.

 

1.5 The primary attack rate is the number of new cases as a proportion of the total susceptible population. The secondary attack rate is the number of additional cases of disease among contacts of the primary or index cases within the maximum incubation period expressed as proportion of the total number of susceptible contacts. The case fatality ratio is number of fatal outcomes expressed as a proportion of the total number of cases with symptomatic illness.

 

2.0 THE AGENT: MICRO-ORGANISM

2.1 Microorganisms can be classified by structure as pro-karyotes (bacteria, rickettsiae, and Chlamydia), eukaryotes (fungi, protozoa, and helminths), and viruses;

 

2.2 Microorganisms can be classified by transmission: as feco-oral transmission (ameba, giardia, shigella, vibrio cholera, salmonella spp, hepatitis virus A&E, polio virus, and tapeworms), soil contact transmission (trichuris, ascaris, hookworms, strongyloides, and tetanus), water contact transmission (schistosomiasis and guinea worm), skin contact transmission  (chickenpox and smallpox), air transmission (measles, pertussis, diphtheria, and tuberculosis), contact with body fluids (trachoma, syphilis, gonorrhea, and HIV), and arthropod-borne (dengue virus and the malarial protozoan).

 

2.3 Microorganisms can be classified by infectivity, virulence, pathogenicity, toxigenicity, and the infective dose.

 

2.4 Microorganisms can be classified as saprophytic (live on dead organic matter), parasitic (depend on and harm the human host), commensals (depend on but do not harm the human host), and symbiotic (mutual benefit). Obligate parasites (eg viruses) cannot exist outside their host. Normal bacterial flora is opportunist pathogens.

 

2.5 Viral infections are recognized by detection of viral antigens or antibodies to viral antigens using serological technics (neutralization, hemagglutination inhibition, complement fixation, fluorescent antibody, radioimmunoassay, and ELISA) and cell culture.

 

2.6 Diagnosis of bacterial infections is based on cultivation & identification, gram staining (+ve and –ve)  & microscopy, biochemical reactions, use of specific antibodies, and DNA hybridization.

 

2.7 Fungal infections are less common. They are transmitted by direct soil contact, inhalation, and rarely person to person. They are localized and are rarely systemic. Diagnosis of fungal infection is difficult and is based on clinical or histological examination.

 

2.8 Protozoa are single-celled eukaryotic organisms widely distributed in the world in both parasitic and free-living forms. The species that are epidemiologically important are plasmodium, toxoplasma, and pneumocytes. Plasmodia cause malaria that is the most important protozoal disease in terms of mortality and morbidity. P. falciparum causes more mortality and morbidity than P.ovale, P. vivax, and P. malariae. Toxoplasma gondi has high prevalence with little clinical disease. Pneumocytes carinii is an opportunistic infection in HIV/AIDS.

 

2.9 Helminths are flat worms (cestodes), flukes (trematodes), and roundworms (nematodes). The most important helminths epidemiologically are schistosomiasis, hookworm, strongyloides, echinococcus, tenia, and toxicara.

 

2.10 Some microorganisms survive in reservoirs until they can infect humans. Common vehicle spread is by water, air or food. Serial transmission is human to human, human to animal to human, or human to environment to human. Infection can be exogenous infections or endogenous infections. Auto-infection found in strongyloides and E.coli. Horizontal transmission (human to another) is more common than vertical transmission (intra-uterine from the mother to fetus ?CMV, ?toxoplasma, ?rubella, ?HSV, ?syphilis, ?TB, ?VZ.).

 

2.11 The natural portals of entry into human are the respiratory tract (common cold, influenza, measles, TB, whooping cough), the urogenital tract (gonorrhoea, syphilis, herpes, HIV), the alimentary tract (amebic dysentery, shigellosis, polio, and cholera), the mucous membranes, the skin, the placenta (rubella, syphilis, and HBV), and the parenteral portal (intravenious and sexual). The skin is a good natural barrier to infection but can be penetrated by insects, ticks, needles, and traumatic injuries.

 

2.12 Air-borne infections are TB, influenza, histoplasmosis, and legionellosis. Vehicle borne transmission is by contaminated materials or objects (fomites). Vectors may be arthropods (such as mosquitoes, fleas, flies, lice, and ticks), zoonoses, plants, or other vehicles. They transmit organisms mechanically (eg flies and cockroaches) or hematophagous (eg ticks, lice, fleas, mites), or biologically. Biological transmission is more common than mechanical transmission. Zoonoses are diseases whose reservoirs are vertebrate animals and are transmitted to humans by accident for example plague, rabies, Rocky Mountain spotted fever. Anthroponooses are diseases whose reservoirs are human for example measles. Plants can be vectors of disease when they are contaminated by micro-organisms and are eaten raw.

 

2.13 The concept of the communicable disease triangle simplifies discussion of communicable disease. The triangle consists of the agent, the host, and the disease (or environment). They interact among one another. Elements of the environment that affect disease transmission are: climate (temperature, rain, and wind patterns), vegetation (swamps, forest, and desert), water sanitation, air pollution, excreta disposal, housing, occupation (farm, factory). Poor sanitation and crowding increase the transmission of microorganisms. Breeding places near homes, forest reservoirs, and soil help the survival of organisms and their vectors.

 

3.0 THE HOST

Humans can be intermediate hosts, definitive hosts, reservoir hosts, and accidental hosts. Humans can also be in a carrier status. They can be healthy, incubational, convalescing, or chronic carriers. Susceptibility to infection is determined by age, heredity, gender, pregnancy, nutritional status, life-style and behavior, personal hygiene, and immune resistance (natural or acquired; passive or active). Immune resistance is the main barrier to infection by micro-organisms.

 

4.0 THE DISEASE

Clinical severity can be described as mild, moderate, severe, and fatal. Clinical manifestations are: asymptomatic disease, latent disease, sub-clinical disease, and clinical disease. The 4 stages of the natural history are pre-pathogenesis, pre-clinical, clinical, and chronic.

 

Action

Period

Infectiousness

Latent Period

Infectious Period

Non-infectious Period

Disease

Incubation Period

Symptomatic Period

No disease Period

 

An epidemic occurs due to changes in the agent, host susceptibility, and effective transmission. The agent may be new, may increase in number, or may change in virulence. An adequate susceptible population is required to sustain and propagate the epidemic.

 

5.0 CONTROL and PREVENTION OF COMMUNICABLE DISEASE

5.1 Control is by identification of cause, notification, treatment of cases using drugs, prevention, and surveillance. The strategy is attacking the agent at source, interrupting transmission, and reducing the susceptible population.

 

5.2 Measures for the healthy host are active immunization, passive immunization, chemoprophylaxis, behavioral change (sexual, dietary), physical isolation, and increase of host resistance by better nutrition and health care.

 

5.3 Measures for the diseased host are chemotherapy, isolation, quarantine, restriction of activity, and behavioral change.

 

5.4 Vector control is by chemicals, environmental and biological control. Control of animal reservoirs is by active immunization, restriction of movement or reduction in number, chemoprophylaxis and chemotherapy.

 

5.5 Environmental control is by water sanitation, safe drinking water, excreta disposal, and food sanitation.

 

5.6 Control of the agent is by cleanliness, refrigeration, disinfection, and sterilization. Disease notification plays a central role in disease control.

 

5.7 The notifiable diseases are: AIDS, anthrax, botulism, brucellosis, cholera, congenital rubella syndrome, diphtheria, encephalitis, gonorrhoea, H. influenzae, Hansen’s disease, leptospirosis, lyme disease, measles, plague, paralytic polio, psittacosis, rabies, syphilis, tetanus, trichinosis, tularemia, typhoid, and typhus.

 

5.8 Primary prevention is prevention of initial contact and/or infection. Non-epidemic secondary prevention consists of diagnosing and treating cases. Secondary prevention requires systematic investigation and taking specific control measures. Tertiary prevention limits chronic disability by physiotherapy, supportive care, and surgical correction of deformities.

 

6.0 EMERGING AND RE-IMERGING DISEASES

6.1 OVER VIEW

The reemergence of infectious diseases in the developed countries after falling over most of the 20th century is due to socio-demographic, lifestyle or human behavior, environmental, and medical technological factors. Some diseases are old diseases and are old problems. Some diseases are old but are new problems for example tuberculosis and malaria. Some are new diseases with new pathogens such as Ebola and HIV.

 

6.2 SEXUALLY TRANSMITTED DISEASES

The traditional STDs are syphilis, gonorrhoea, and chanchroid. New diseases are chlamydia, genital warts, trichomonas, scabies, peduculosis, genital herpes, vaginal candidiasis, E. histolytica infection, G. lamblia, HVA, HBV, HCV, and HIV.

 

6.3 VIRAL DISEASES

Emerging diseases are the Ebola/Marburg virus epidemic started in 1976, the swine flu epidemic recognized in 1976, and Lassa fever. Traditional diseases with increasing transmission are dengue fever, dengue hemorrhagic fever, Hepatitis B, hepatitis C, Rift valley fever and  Yellow fever.

6.4 BACTERIAL DISEASES

Emerging diseases are the toxic shock syndrome due to infection of ultra absorbent tampons by Staphylococcus aureus in 1980. re-emerging diseases are: cholera, Escheria coli, Helicobacter Pylori, and tuberculosis

6.5 PARASITIC DISEASES:

Emerging diseases: Lyme disease and Legionnaire’s disease

Re-emerging diseases: Malaria and schistosomiasis is spreading due to dam building.

 

ISSUES IN COMMUNICABLE DISEASES

DETAILED NOTES

 

1.0 EPIDEMIOLOGICAL CHARACTERIZATION

A. DEFINITIONS

Infectious disease is disease transferred from person to person by micro-organisms. Infectious disease results from the action of the infectious agent or its products. The terms communicable disease and infectious disease are synonymous. Infection is the process of lodging, growing, and multiplication of micro-organism in host’s body. Invasion is mere presence of micro-organism in body of host without necessarily multiplying. Infectivity is the ability of the infective agent to lodge and grow in the host. Pathogenicity is the ability of the organism to cause disease and is measured as the proportion of the number of infected persons with clinical disease to the total number of infected person. Virulence is the ability to cause severe disease. The measles and varicella zoster viruses are very pathogenic but not virulent. HIV is very virulent since all those affected eventually die. Pathogenicity is affected by invasiveness, toxigenicity, and hypersensitivity. Some organisms like shigella invade tissues. Others like Cl. Botulinum produce toxins. Mycobacterium tuberculosis causes hypersensitivity or allergic reactions in the host. An epidemic is said to occur when the epidemic threshold is breached. The term epidemic is used to refer to a wide-spread disease whereas the term outbreak is used to refer to localized disease.

 

B. COMMUNICABLE DISEASE MODEL

THE TRIAD

The communicable disease model comprises of the three minimum factors needed for occurrence and spread of disease: the agent, the host, and the environment. The environment, physical or biological, may inhibit or promote disease transmission. Disease transmission occurs when a susceptible host and a pathogenic agent exist in an environment conducive to disease transmission.

                                                  

GASTRIC INFECTION – PEPTIC ULCER

Peptic ulceration is a problem in both developed and less developed countries. The disease is a result of interplay of genetic and environmental factors. Death from PU complications rises with age. H. pylori infection and use of non-steroidal anti-inflammatory drugs are factors determining severity of the disease. Smoking increases the risk of PU. There is no evidence for the role of diet or stress in PU. H. pylori infection is found in association with PU. Its eradication leads to subsiding of PU symptoms. PU infection is acquired in childhood due to poor sanitation. Most of those infected do not develop the ulcer. The risk of H.pylori infection is falling in Europe and America.

 

INTESTINAL INFECTION

Intestinal infections present as watery diarrhoea, bloody diarhoea (dysentery), or chronic diarrhoea with or without steatorrhoea. Watery diarrhoea is self limiting and resolves within 5-7 days. It requires only fluid and electrolyte replacement. Dysentery and chronic diarrhoea require specific treatment. The prevalence of intestinal infections is higher in less developed countries because of poor sanitation, water shortage, poor hygiene, and crowding. Intestinal infections still accounts for substantial morbidity in developed countries. Intestinal infection in the US is on the increase. Most intestinal infections are self limiting and a very small proportion is notified. Children bear the heaviest burden of intestinal infections.

 

The following are factors in intestinal infection: travel, immunodeficiency, institutional living, day care, reduced gastric acid secretion, extremes of age, swimming, eating raw fish, eating partially cooked eggs, and eating myonnaise. Intestinal infections are transmitted by the feco-oral route. Humans are the major reservoirs of intestinal infections. Salmonella spp and Cl. Jejuni are exceptions because they have animal reservoirs. The majority of bacterial enteropathogens have seasonality. Cholera occurs more in the rainy season. Parasites occur more in winter because their cysts can survive better in the cold climate. Ritavirus infection is more common in the winter. Intestinal infections are prevented by interrupting the fecal-oral transmission route. This is achieved by using clean potable water, safe fecal disposal, personal hygiene, and food hygiene. Propyhllatic antibiotics can be used at half the therapeutic dose. Vaccination can be carried out for cholera, salmonella spp. and shigella spp. The evidence for probiotics in prevention is not definitive.

 

C. RATES & PROPORTIONS

The primary attack rate is defined as the number of new cases of disease expressed as a proportion of the total susceptible population. The secondary attack rate is defined as the number of additional cases of disease among contacts of the primary or index cases within the maximum incubation period expressed as proportion of the total number of susceptible contacts. The case fatality ratio is number of fatal outcomes expressed as a proportion of the total number of cases with symptomatic illness.

 

D. EPIDEMIOLOGY

Correct and complete incidence and prevalence on communicable diseases is not available for many diseases. This is due to incomplete reporting and non-detection of sub-clinical cases.

 

E. TRENDS OF INCIDENCE

Most morbidity & mortality in the world is due to microbials. Most of this mortality and morbidity is in the less developed countries of Asia, Africa, and Latin America. Infection patterns differ according to level of economic development. Less developed countries have higher incidence and prevalence of infectious diseases than the developed countries. Most less developed countries are experiencing a falling incidence of infection and mortality due to socio-economic improvement, specific and non-specific primary prevention (sanitation and immunisation).  In both LDC and industrialized countries, old diseases like small poxes are being controlled. Some old ones like TB and syphilis are re-emerging. New diseases related to lifestyle like HIV and other STDs are appearing and are increasing.

Omar Hasan Kasule, Sr, Sep 2005