COMMON VEHICLE SPREAD

The term common vehicle spread describes the mode of transmission of infectious pathogens from a source that is common to all the cases of a specific disease, by means of a medium, or "vehicle," such as water, food, air, or the blood supply used by a transfusion service. Communicable diseases that are spread in this way do not characteristically manifest themselves in an epidemic of explosive onset with large numbers of cases all occurring at once. Instead, there is a steady or continuous occurrence of cases over a period that may last for weeks, months, or even years until the source of the infection or its mode of transmission is controlled. Cases may occur sporadically, and perhaps infrequently, especially if there is a change in the nature of the vehicle. When water is the vehicle through which pathogenic organisms are disseminated, the frequency of cases may be low and intermittent if the water source is large and contamination occurs only from time to time, perhaps when rains flush pathogens into the water from cattle pastures.

Sometimes infections occur in very large numbers. A water-borne outbreak of cryptosporidiosis in Milwaukee, Wisconsin, in 1993 affected about 400,000 people. The common vehicle for diarrheal diseases was alliteratively summarized in an otherwise somber British War Office handbook on military hygiene in 1914: "Careless carriers, contact cases, chiefly cooks, dirty drinking water, the dust of dried dejecta, and the repulsive regurgitation, dangerous droppings, and filthy feet of fecal-feeding flies fouling food." Milk was a common vehicle for tuberculosis, scarlet fever, and sometimes typhoid fever until pasteurization rendered milk safe and rules for food safety mandated tuberculosis-free cows. The best known airborne common-vehicle infection is Legionnaires' disease, which is spread by moist air such as that from a poorly maintained air conditioning system or the steamy humid air in Turkish baths.

One of the great medical disasters of the late twentieth century occurred when supplies of blood and blood products for transfusion services in many nations were contaminated by HIV (human immunodeficiency virus) and hepatitis viruses. The principle victims were hemophiliacs being treated with factor VIII and patients undergoing open-heart surgery or other major operations that required the transfusion of multiple units of blood.