Disease Transmission

Written by Ann Pietrangelo
Medically Reviewed by George Krucik, MD, MBA on May 30, 2013

Disease Transmission Overview

Infectious diseases are those that are transmitted from person to person by direct or indirect contact. Viruses, bacteria, parasites, and fungi all cause infectious disease. Malaria, measles, and respiratory illnesses are examples of infectious diseases. 

Simple preventative measures, like frequent hand washing, can cut down on disease transmission.

Direct Contact

Infectious diseases are often spread through direct contact. 

Person-to-Person Contact

Infectious diseases are most commonly transmitted through direct person-to-person contact. Transmission occurs when an infected person touches or exchanges body fluids with someone else. This can happen before a person is aware that they are ill. Respiratory illnesses and sexually transmitted diseases (STDs) are some of the diseases that can be transmitted this way.

Pregnant women can also transmit some infectious diseases to their unborn children via the placenta. Some STDs, including gonorrhea, can be passed from mother to baby during childbirth.  

Droplet Spread

The spray of droplets during coughing and sneezing can spread infectious disease. You can even infect another person through the droplets created when you speak. Droplets fall to the ground within a few feet, so this type of transmission requires close proximity.

Indirect Contact

Infectious diseases can also be spread indirectly through the air and other mechanisms.

Airborne Transmission

Some infectious agents can travel long distances and remain suspended in the air for an extended period of time. You can catch a disease like measles by entering a room well after someone with measles has departed.

Contaminated Objects

Some organisms can live on objects for a short time. If you touch an object, such as a doorknob, soon after an infected person, you are exposed to infection. Transmission occurs when you touch your mouth, nose, or eyes before thoroughly washing your hands.

Germs can also be spread through contaminated blood products and medical supplies.

Insect Bites (Vector-borne Disease)

Some infectious agents are transmitted by insects, especially those that suck blood. These include mosquitos, fleas, and ticks. The insects become infected when they feed on infected hosts, such as birds, animals, and humans. The disease is then transmitted when the insect bites a new host. Malaria, West Nile virus (WNV), and Lyme disease are all spread this way. 

Food and Drinking Water

Infectious diseases can be transmitted via contaminated food and water. E. coli is often transmitted through improperly handled produce or undercooked meat. Improperly canned foods can create an environment ripe for Clostridium botulinum, which causes botulism. 

Animal-to-Person Contact

Some infectious diseases can be transmitted from an animal to a person. This can happen when an infected animal bites or scratches you, or when you handle animal waste. The Toxoplasma parasite can be found in cat feces. Pregnant women and people with compromised immune systems should take extra care (disposable gloves and good hand washing) when changing cat litter, or avoid it altogether.

Animal Reservoirs

Animal-to-animal disease transmission can sometimes transfer to humans. Zoonosis occurs when diseases are transferred from vertebrate animals to people. Zoonotic diseases include anthrax (from sheep), rabies (from rodents and other mammals), and WNV (from birds). Plague is transmitted through rodents.

Environmental Reservoirs

Soil, water, and vegetation containing infectious organisms can also be transferred to people. Hookworm, for example, is transmitted through contaminated soil. According to the United States Centers for Disease Control and Prevention (CDC), Legionnaires’ disease is an example of a disease that can be spread by water that supplies cooling towers and evaporative condensers (CDC, 2013).

How to Prevent Disease Transmission

A few simple precautions can prevent some disease transmission. The most important of these is to wash your hands thoroughly and often.


When you have a contagious illness, try to avoid direct contact with other people. Cover your nose and mouth when you sneeze and cough. When caring for an ill person, use disposable gloves and wash your hands frequently. 

Foodborne Illness

Dangerous organisms can thrive in improperly prepared food. Avoid cross-contamination by keeping raw meats and produce separate. Use different preparation surfaces and wash surfaces and utensils thoroughly. 

Freeze or refrigerate perishable foods and leftovers promptly. According to the United States Department of Agriculture (USDA), you should set your refrigerator to 40°F or below and your freezer to 0°F or below. Cook meats to a minimum internal temperature of 145°F. Cook ground meats to 160°F and poultry to 165°F (USDA, 2011).

Be careful about sources of food when visiting foreign countries.

Insects and Animals

When camping or enjoying wooded areas, wear long pants and long sleeves. Use insect repellent and mosquito netting. Don't touch animals in the wild.


Stay up to date on vaccinations, especially when traveling. Don't forget to keep your pet's vaccinations current, too.

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