Lesson 1: Introduction to Disease Transmission and Epidemiology

1-8

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1-8. MEANS OF DISEASE TRANSMISSION (MODE OF TRANSMISSION)

a. Physical (Direct) Contact. Certain diseases are spread by physical contact with an infected person. Examples are syphilis, gonorrhea, and scabies.

b. Indirect Contact.

(1) Droplets, air, and dust. Droplets are vehicles by which a disease may be transmitted from an infected person to susceptible persons. When an infected person coughs, sneezes, or even talks, he spreads droplets containing disease germs. If other persons are close to the infected person, they may inhale some of these droplets. Furthermore, some germs expelled from the respiratory tract are extremely small and light in weight and may remain suspended in the air for hours or may be resuspended in dust. Inhalation of these germs by susceptible persons may also result in disease. Many of the respiratory diseases are transmitted in these ways.

 

(2) Fomites. Articles contaminated with disease germs from an infected person may become vehicles of disease transmission if a susceptible person uses them. Examples of fomites are contaminated clothing, bed linen, and eating utensils.

c. Arthropods or Other Vectors. Flies, fleas, mosquitoes, ticks, mites, and lice are among the arthropods that spread disease from person to person or from animal to person. Insects are involved in both the direct and the indirect transmission of disease. A mosquito, for example, can pick up disease germs when it bites a person sick with a disease such as malaria. Later, when the mosquito bites another person, it injects the disease germs. The mosquito is, therefore, the vector by which the disease is transmitted from one person directly to another person. The fly, on the other hand, transmits disease germs indirectly. It can pick up disease germs on its body when it comes in contact with filth and may deposit these germs on food. If a person eats this food, he may become ill.

d. Water and Food. Certain disease germs are transmitted through the consumption of foods such as raw fish and improperly cooked meat and poultry. However, most of the diseases that are transmitted by food and water are the result of contamination of the food or water with feces or other infectious material from a person or animal. If water or food so contaminated is not properly treated, the germs therein may infect the consumer. Outbreaks of disease will occur when personal hygiene and proper sanitation practices applicable to food handling, water purification, water disposal, and the control of flies and other vermin are not properly observed and enforced. Among the diseases usually transmitted by contaminated food or water are typhoid fever, infectious hepatitis, cholera, dysentery, and food poisoning (due to staphylococci, Clostridium perfringens, and other organisms.)

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