Taking Our Temperature

How Diseases Get Spread

Diseases get around in two ways. Some travel directly from one person to another. Others rely on a “go-between” for a lift.

Biotic Factors: These relate to living things. If the rat population in a city grows, diseases carried by rats have a better chance of spreading.

Abiotic Factors: These have to do with nonliving parts of the environment, such as temperature, sunlight, and chemicals.
A cold wave may kill off mosquitoes—and the diseases they carry.

Biotic and abiotic factors often affect each other. In 1993 the southwestern United States received more rain (abiotic) than usual. Plants (biotic) grew with gusto. That meant more food for mice (biotic). The mouse population ballooned, spreading a hantavirus that makes humans seriously ill.

Knowing how disease spreads is a crucial starting point for thinking about global warming and human health. The more we understand about disease, the better chance we have at preventing or minimizing outbreaks related to climate change. Diseases basically have two simple—but all too effective—techniques for transmission, or spreading.

Direct Transmission: That's when disease-causing microbes (alias "germs") travel straight from one person to another. Let's say, for instance, that your kid brother catches a cold. He unintentionally sneezes in your face while you're wrestling for the remote. Vicious little viruses land right on your skin. Before long, Bro' has the remote, and you have the cold.


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Health officials may quarantine, or block off, places. Source: CDC
In extreme cases, health officials may quarantine, or block off, places where a contagious disease has appeared.
Source: Center for Disease Control and Prevention


Indirect Transmission: Even little brothers eventually learn how to use tissues, so diseases have other ways to get around too. Some bacteria and viruses hang out in water or soil. When people drink the water that's contaminated with microbes, they get sick. Many diseases hitch rides aboard vectors — science lingo for insects and other living things that carry bacteria, viruses, or parasites.


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Chart of Global Microbial Threats in the 1990s. Source: CDC
This map shows outbreaks of microbe-caused diseases during the 1990s.
Source: Center for Disease Control and Prevention


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