How Diseases Spread

How are infectious diseases spread?

Infectious diseases can be spread in a number of ways. Some diseases are spread through coughing, sneezing, physical contact, or exchange of body fluids. Some diseases are spread through contaminated water or food. Still other diseases are spread to humans from infected animals.

Pathogens are often spread by symptoms of disease, such as sneezing, coughing, or diarrhea. In many cases, these symptoms are changes in host behavior that help pathogens spread and infect new hosts! After all, if a virus infects only one host, that virus will die when the host's immune system kills it or when the host dies. For that reason, natural selection favors pathogens with adaptations that help them spread from host to host.

FIGURE 35–2 Sneezing Some infectious diseases are spread from person to person by sneezing. Thousands of pathogen particles can be released in a sneeze. Infer Why is it more beneficial to sneeze into a tissue rather than covering your mouth with your hand?

Coughing, Sneezing, and Physical Contact Many bacteria and viruses that infect the nose, throat, or respiratory tract are spread by indirect contact. Coughing and sneezing releases thousands of tiny droplets that can be inhaled by other people. Those droplets also settle on objects such as doorknobs. If you touch those objects and then touch your mouth or nose, you can transfer the pathogens to a new home! Thus, the ability of a flu virus or a tuberculosis bacterium to cause a host to sneeze or cough is an adaptation that increases transmission of the pathogen from one host to another.

Other pathogens, including drug-resistant staphylococci that cause skin infections, can be transferred by almost any kind of body-to-body contact. They can also be transferred by contact with towels or certain kinds of sports equipment.

Minimizing transmission of these diseases is surprisingly simple. The most important means of infection control is thorough and frequent hand washing. If you have a cold or flu, cover your mouth with a tissue when you cough or sneeze, and wash your hands regularly.

Exchange of Body Fluids Some pathogens require specific kinds of direct contact to be transferred from host to host. For example, a wide range of diseases, including herpes, gonorrhea, syphilis, and chlamydia, are transmitted by sexual activity. Therefore, these diseases are called sexually transmitted diseases. Other diseases, including certain forms of hepatitis, can be transmitted among users of injected drugs through blood from shared syringes. HIV can be transmitted through blood or sexual contact. Sexually transmitted diseases can only be completely prevented by avoiding sexual activity.

Contaminated Water or Food Many pathogens that infect the digestive tract are spread through water contaminated with feces from infected people or other animals. Symptoms of these diseases often include serious diarrhea. This is another adaptation that helps pathogens spread from one host to another, especially in places with poor sanitation.

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