DISEASE RESERVOIR That's the number of people or animals carrying the disease-producing microbe. Bigger reservoirs mean more chances for vectors to pick up and spread the illness. POPULATION DENSITY When people spread out, vectors have a harder time infecting them. When people are packed together—as in refugee settlements or cities—vectors can infect a large population fairly easily. TEMPERATURE AND RAINFALL Changes in weather can make certain places more hospitable to vectors—and the diseases they carry.

Think of vectors as the taxis of the disease world. Bacteria, viruses, and parasites ride from one person (or animal) to another. As the vector goes about its life, the disease spreads. Vector-borne diseases are some of the most deadly and unpredictable medical problems that humans face.

Most vectors are bloodsucking arthropods—animals with external skeletons, segmented bodies, and jointed limbs. "Superstar" vectors include fleas, mites, ticks, and (of course) mosquitoes. Lapping blood from animals or humans, these creatures also pick up disease-causing microbes or germs. The vector doesn't get sick. But it does transfer the microbes to the next person it bites. The microbes then infect that person.

Malaria-spreading mosquitoes are a notorious example of vectors. Source: World Health Organization

One of the most vicious vector-borne diseases of all time is the plague, more colorfully known as the Black Death. In just six years (1347-52), bacteria killed a quarter of Europe's population. Fleas carried the plague from rats to humans.

Infected by the bacterium that causes plague, this flea could spread that dreaded disease to rats and humans. Source: Center for Disease Control and Prevention

How will global warming affect vector-borne diseases? Scientists say it's difficult to know for sure. One big concern is that rising temperatures could affect rainfall patterns. Some places may grow warmer and wetter—and perhaps more prone to disease.

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Rising temperatures during the 20th century altered precipitation around the globe. Source: United Nations Environment Programme

In the U.S., for instance, encephalitis (brain swelling caused by a mosquito-borne virus) thrives when wet, warm winters give way to hot, dry summers. In the tropics, malaria epidemics often follow the rainy season. More rain could mean more malaria.

Then again, high temperatures could kill off tick or mosquito vectors in some places.
We just don't know.

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Boxes in the far-right column show whether scientists consider it likely (green), very likely (yellow), or highly likely (red) that global climate change will affect the spread of each disease. Source: United Nations Environment Programme

There's a way to find clues, however. That's by looking carefully at disease case studies, such as malaria. Seeing how climate affects each disease can help scientists hypothesize about how other diseases may respond to climate changes.

The links below take you to three case studies:

Malaria Dengue Fever Hantavirus Pulmonary Syndrome