The process by which a disease is transmitted via micro-organisms from one person to another. The micro-organism may be a bacterium (see BACTERIA), RICKETTSIA, a VIRUS, or a protozoon (single-celled animal organism). Invasion of the body by a metazoon (multicellular animal organism), e.g. an intestinal worm, is more often called an infestation.
The skin is an important protection against micro-organisms entering the body tissues. A large measure of protection is afforded by the factors which ensure IMMUNITY against diseases.
The infective material may be transmitted to the person by direct contact with a sick person, when the disease is said to be contagious. Infection may be conveyed on dust, in drinking-water, in food (particularly milk), in the body's waste products and secretions, or even on clothes and linen which have been in contact with the infected individual (called fomites). Different diseases are especially infectious at different periods of their course. Protecting people can be difficult, since some diseases are infectious before the patient shows any symptoms (see INCUBATION).
Some people who have recovered from a disease, or who have simply been in contact with an infectious case, harbour the infectious agent (carrier state). This is particularly the case in typhoid fever (see ENTERIC FEVER), the bacillus continuing to develop in the gall-bladder of some people who have had the disease for years after the symptoms have disappeared. In the case of CHOLERA, which is ENDEMIC in some developing countries with hot climates, 80 per cent or more of the population may harbour the bacillus and spread infection when other circumstances favour this. Similarly in the case of DYSENTERY, people who have completely recovered may still be capable of infecting dust and drinking-water by their stools. DIPHTHERIA and meningococcal MENINGITIS, which is particularly liable to infect children, are other examples.
Flies can infect milk and other food with the organisms causing typhoid fever and food poisoning. Mosquitoes carry the infective agents of MALARIA, DENGUE and YELLOW FEVER, these undergoing part of their development in the body of the mosquito. Fleas convey the germ of plague from rats to humans, and lice are responsible for inoculating TYPHUS FEVER and one form of RELAPSING FEVER by their bite. A tick is responsible for spreading another form of relapsing fever, and kala-azar (LEISHMANIASIS) is spread by the bites of sandflies.
Certain of the common and most serious infectious diseases are notifiable in the United Kingdom. A doctor diagnosing someone infected by a notifiable disease must inform the authorities. For the current list of notifiable infectious diseases, see the main entry for NOTIFIABLE DISEASES.
Prevention is an important aspect of the control of infectious diseases, and various steps can be taken to check the spread of such infections as dysentery, tuberculosis, malaria and others. (See also IMMUNITY; INCUBATION.)