FAQ: Methods of Disease Transmission

• What should I know about disease transmission?
• What is transmission by direct contact?
• What is transmission by indirect contact?
• What is transmission by droplet contact?
• What is airborne transmission?
• What is fecal-oral transmission?
• What is vector-borne transmission?

» What should I know about disease transmission?

Knowing the methods in which a disease is transmitted is important for implementing proper infection control measures and large scale prevention campaigns. Each disease has transmission characteristics based on the nature of the microorganism that causes it.

The types of transmission described below are not mutually exclusive. Some diseases, such as anthrax, can be transmitted in more than one way. Anthrax can be spread through direct contact to a cut on the skin, producing cutaneous anthrax. It can also be spread through airborne spores which are inhaled, producing a more serious type of infection. Gastrointestinal anthrax can occur when anthrax spores are ingested.

» What is transmission by direct contact?

Direct contact transmission requires physical contact between an infected person and a susceptible person, and the physical transfer of microorganisms. Direct contact includes touching an infected individual, kissing, sexual contact, contact with oral secretions, or contact with body lesions. This type of transmission requires close contact with an infected individual, and will usually occur between members of the same household or close friends and family.

Diseases spread exclusively by direct contact are unable to survive for significant periods of time away from a host. Sexually transmitted diseases are almost always spread through direct contact, as they are extremely sensitive to drying.

» What is transmission by indirect contact?

Indirect contact transmission refers to situations where a susceptible person is infected from contact with a contaminated surface. Some organisms (such as Norwalk Virus) are capable of surviving on surfaces for an extended period of time. To reduce transmission by indirect contact, frequent touch surfaces should be properly disinfected.

Frequent touch surfaces (fomites) include:

• Door knobs, door handles, handrails
• Tables, beds, chairs
• Washroom surfaces
• Cups, dishes, cutlery, trays
• Medical instruments
• Computer keyboards, mice, electronic devices with buttons
• Pens, pencils, phones, office supplies
• Children's toys

» What is transmission by droplet contact?

Some diseases can be transferred by infected droplets contacting surfaces of the eye, nose, or mouth. This is referred to as droplet contact transmission. Droplets containing microorganisms can be generated when an infected person coughs, sneezes, or talks. Droplets can also be generated during certain medical procedures, such as bronchoscopy. Droplets are too large to be airborne for long periods of time, and quickly settle out of air.

Droplet transmission can be reduced with the use of personal protective barriers, such as face masks and goggles. Measles and SARS are examples of diseases capable of droplet contact transmission.

» What is airborne transmission?

Airborne transmission refers to situations where droplet nuclei (residue from evaporated droplets) or dust particles containing microorganisms can remain suspended in air for long periods of time. These organisms must be capable of surviving for long periods of time outside the body and must be resistant to drying. Airborne transmission allows organisms to enter the upper and lower respiratory tracts. Fortunately, only a limited number of diseases are capable of airborne transmission.

Diseases capable of airborne transmission include:

• Tuberculosis
• Chickenpox
• Measles

» What is fecal-oral transmission?

Fecal-oral transmission is usually associated with organisms that infect the digestive system. Microorganisms enter the body through ingestion of contaminated food and water. Inside the digestive system (usually within the intestines) these microorganisms multiply and are shed from the body in feces. If proper hygienic and sanitation practices are not in place, the microorganisms in the feces may contaminate the water supply through inadequate sewage treatment and water filtration. Fish and shellfish that swim in contaminated water may be used as food sources. If the infected individual is a waiter, cook, or food handler, then inadequate handwashing may result in food being contaminated with microorganisms.

Fecal-oral transmission can be reduced by:

• Proper storage of food at proper temperatures
• Thorough cooking of food
• Frequent and thorough handwashing, especially after washroom use
• Adequate sewage treatment and water filtration/chlorination systems
• Disinfection of frequent touch surfaces to prevent indirect contact transmission
• Increased public awareness of proper hygiene and food handling

» What is vector-borne transmission?

Vectors are animals that are capable of transmitting diseases. Examples of vectors are flies, mites, fleas, ticks, rats, and dogs. The most common vector for disease is the mosquito. Mosquitoes transfer disease through the saliva which comes in contact with their hosts when they are withdrawing blood. Mosquitoes are vectors for malaria, West Nile virus, dengue fever, and yellow fever.

Vectors add an extra dimension to disease transmission. Since vectors are mobile, they increase the transmission range of a disease. Changes in vector behaviour will affect the transmission pattern of a disease. It is important to study the behaviour of the vector as well as the disease-causing microorganism in order to establish a proper method of disease prevention. In the case of malaria, insecticides were sprayed and breeding grounds for mosquitoes were eliminated in an attempt to control the spread of malaria.

Biting is not the only way vectors can transmit diseases. Diseases may be spread through the feces of a vector. Microorganisms could also be located on the outside surface of a vector (such as a fly) and spread through physical contact with food, a common touch surface, or a susceptible individual.