The prevalence of infectious diseases varies markedly throughout the world and depends on climatic conditions, sanitation, the quality of the water supply, and to some extent the specific disease resistance of the indigenous population at risk. The continuance of infectious diseases in a human population requires:\
• Reservoirs of infection
• Effective modes of transmission
HUMAN RESERVOIRS are necessary for the agents of those diseases that (under natural conditions) exclusively afflict humans. Specific examples of such diseases are hepatitis A and B, cholera and shigellosis. Many sites in the body act as permanent reservoirs for microorganisms:
• Skin, e.g. Staphylococcus epidermidis
• Nasopharynx, e.g. meningococci
• Intestinal tract, e.g. Giardia, Entamoeba histolyticaboth can continue to colonize after clinical recovery
Viruses may remain in the body for many months or years, notable examples being hepatitis B virus and the neurotropic herpesviruses.
Helminths may remain in the circulation (e.g. schistosomes in the portal vein) or lymphatic system (e.g. filarial worms) for many years, the former constantly producing millions of ova, a high proportion of which are deposited back into the environment.
ANIMAL RESERVOIRS of human disease are also important both in the developed and developing worlds. The following are common examples of zoonoses (infections that can be transmitted from animals, except arthropods, to man):
• From battery-farmed chickens-Salmonella or Campylobacter jejuni infection
• From domestic cats- Toxoplasma gondii infection
• From domestic and wild animals – Giardia infection
• From cattle-Cryptosporidium parvum infection
Diseases that rely on arthropods for their transmission include malaria, yellow fever, Dengue fever and rickettsial infections.
ENVIRONMENT RESERVOIRS may also act as a temporary lodging place for some bacteria, viruses and parasites.
Water contaminated with enteropathogens is a constant cause of concern in the tropics; water may also be a reservoir of hepatitis A virus. Cysts of some protozoa, notably Giardia, may remain viable despite apparently effective water-purification procedures.
Soil is also a source of the agents of human disease, particularly spore-forming bacteria such as Clostridium spp. and Bacillus anthracis, whose spores can remain viable under suitable climatic conditions for many months.
AIRBORNE SPREAD. Some viruses, bacteria and bacterial spores can be carried directly by the wind. Some are generally spread by droplets in the air, e.g. influenza viruses, and other microorganisms such as Legionella are spread by aerosol, characteristically from air-conditioning units.
SPREAD BY DIRECT CONTACT. This includes:
• Person-to-person spread, e.g. skin infections (impetigo, ringworm and scabies) and sexually transmitted diseases
• Faecal-oral spread, particularly amongst children in residential institutions, e.g. shigellosis, giardiasis and hepatitis A
• Inoculation of infection, e.g. transfusion of blood or blood products containing hepatitis B, C or HIV or by contaminated needles (drug abusers, medical and paramedical personnel)
• Insect bites, e.g. mosquitoes (leishmaniasis), ticks (babesiosis)
• Entry through the skin, which occurs with the larval forms of some helminths that can survive in soil or water, e.g. Schistosoma, Strongyloides and hookworm (malaria), sandfly and bugs (Chagas’
SPREAD BY FOOD AND WATER. Contaminated food and water is the usual mode of transmission of enteropathogens. Some bacteria, such as Shigella, require as few as 102 organisms to initiate infection, whereas others like Vibrio cholerae require approximately 108 organisms.
Cysts of parasites such as Giardia and Entamoeba histolytica can survive in water for many months and are relatively resistant to water-treatment procedures. Swimming pools are also recognized to be a source of these parasites.
SPREAD BY FOMITES. Transmission of infection can occur between persons via an inanimate object, e.g. bed linen, books.