The terrifying link between climate change and infectious disease, in one chart

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Everyone in the US loves the onset of warm weather, but possibly no one more than ticks, mosquitos, and fleas. For these insects and arachnids, dormant or inactive during during the winter, warm weather means more time to be hunting for food—namely, animal and human blood as we spend more time outside.

Longer spring, summer, and fall seasons due to rising temperatures globally means even more time for the tiny creatures to thrive. They have more opportunities to spread the viruses, bacteria, and parasites that cause diseases  like Zika, Lyme, West Nile, yellow fever, malaria, Chikungunya, and even the plague (in the case of fleas).

And spread they have. A report published yesterday (May 1) by the US Centers for Disease Control shows a three-fold increase in the annual number of vector-borne disease cases reported in the US from 2004 to 2016. In 2016, there were over 96,000 cases, accounting for almost 15% of total reported instances in those 13 years.

Over half of the 2016 cases of vector-borne diseases were spread by ticks, 82% of which were Lyme disease. Most were clustered in the northeast coast, although there were also some cases that popped up in California, Texas, and parts of the midwest and southern states. The 2016 outbreak of Zika in Puerto Rico and the US Virgin Islands made up the majority of the spike in mosquito-borne illnesses.

Warmer temperatures are partially to blame for the spread of these pests, and in some cases even contribute to the likelihood of contracting a disease. In high-temperature environments “the amount of virus in the mosquito increases, and when it bites you, more virus gets into you and the chances of you getting infected and becoming sick goes up,” Lyle Petersen, director of the CDC’s division of vector-borne diseases, told the Washington Post (paywall).

The CDC’s numbers likely represent a low estimate. If a person never felt sick enough to visit a healthcare provider, their case wouldn’t have been recorded. For example, although the Zika virus can cause devastating birth defects, pregnant mothers or other adults who become infected with the disease typically only experience mild symptoms.

Still, not all of these diseases are mild. Lyme disease’s often-mysterious symptoms make it hard to diagnose and can have lingering effects that last for years after the bacteria that causes it has been cleared. Some less-common vector-borne diseases, like St. Louis encephalitis virus, can be fatal or cause life-long paralysis. The only vector-borne disease for which there is a vaccine is yellow fever, of which there was just one case in the entire study period in 2016.