A communicable disease is an illness caused by a specific infectious agent or its toxic products. It arises through transmission of that agent or its products from an infected person, animal, or inanimate reservoir to a susceptible host, either directly or indirectly (through an intermediate plant or animal host, vector, or the inanimate environment). Control of disease is the reduction of disease incidence, prevalence, morbidity, or mortality to a locally acceptable level as a result of deliberate efforts; continued intervention measures are required to maintain the reduction. Control is to be contrasted with elimination (reduction to zero of the incidence of a specified disease in a defined geographic area as a result of deliberate efforts; continued intervention measures are required), eradication (permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts; intervention measures are no longer needed), and extinction (the specific infectious agent no longer exists in nature or the laboratory).
Various categorizations of means of transmission have been used. The American Public Health Association uses these categories: direct transmission, indirect transmission, and airborne. Direct transmission refers to direct contact such as touching, biting, kissing, or sexual intercourse, or the direct projection of droplet spray into the eye, nose, or mouth during sneezing, coughing, spitting, singing, or talking. This projection usually is limited to a distance of 1 meter or less. Examples of direct contact transmission include rabies and sexually transmitted HIV (human immunodeficiency virus). Direct projection is responsible for transmission of diseases such as measles and influenza.
Indirect transmission may occur through a vehicle or an arthropod vector. The causative agent may or may not multiply or develop in or on the vehicle. Examples of possible vehicles include water, food, biological products, or contaminated articles (such as syringe needles). Water-and foodborne diseases have the potential for causing outbreaks involving thousands of persons. Before the causative agent was identified, many cases of HIV resulted from blood transfusion. Since all donor blood in the United States is now screened for HIV, this is no longer a significant means of transmission. However, sharing of needles by injection drug users remains an important factor in the AIDS (acquired immunodeficiency syndrome) epidemic. Arthropod vectors can spread disease mechanically (as a result of contamination of their feet or passage of organisms through the gastrointestinal tract) or biologically (in which the agent must multiply or go through one or more stages of its life cycle before the arthropod becomes infective). Mechanical spread by arthropod vectors is uncommon. However, arthropod-borne diseases such as malaria (in which the parasite develops within the mosquito vector) are still responsible for millions of cases and hundreds of thousands of deaths each year in tropical countries.
Some infectious agents can be spread through the air over long distances. Airborne spread requires that infectious particles are small enough to be suspended in the air and inhaled by the recipient. Tuberculosis and histoplasmosis are bacterial and fungal diseases spread in this fashion. Airborne transmission could also be used to disseminate agents of biological warfare or bioterrorism. Anthrax and smallpox have been considered among the most likely biological weapons.