Taking Our Temperature

Hitching a Ride to Find the Host with the Most


Vectors are critters that transport bacteria, viruses, and parasites. Case studies may offer clues to how global warming will affect vectors—and the deadly illnesses they spread.

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.

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.

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. Source: WHO
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.


Flea infected with the plague bacterium. Source: CDC
Infected by the bacterium that causes plague, this flea could spread that dreaded disease to rats and humans.
Source: World Health Organization


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.


click to enlarge image
Rising temperatures during the 20th century altered precipitation around the globe. Source: UNEP
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.


click to enlarge image
Global climate change and its affect on the spread of diseases. Source: UNEP
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 mate changes.

The links below take you to three case studies:



Additional information