The Centers for Disease Control and Prevention (CDC) recently reported that Lyme disease has significantly expanded in the United States over the past few decades. There are now 17 states considered at high risk in the Northeast and upper Midwest.

Lyme disease is most commonly found in Wisconsin and on the East Coast, especially in Connecticut. But the illness has spread to new areas and increased in incidence in some places. The number of counties with high incidence doubled over a decade, from 130 to 260 counties.

According to CDC, state health departments and the District of Columbia report approximately 30,000 cases of the disease each year. But CDC estimates that the true number of people diagnosed with Lyme disease each year in the United States is around 300,000, since many cases go unreported.

Currently, there is no definitive explanation for the growing threat. However, scientists have a few theories.

Some attribute the expansion of Lyme disease to the growth of the deer population over the past century. With reduced predators and hunting, there are more deer for ticks to feed on. But others argue that this theory is not well-supported, since deer counts have fallen during the time of the Lyme disease expansion.

Another proposed reason for the spread and rise of the disease is climate change. Increasing temperatures create longer summers, and more time for tick larvae to feed before going to sleep in the winter.

Some scientists have also suggested that the rise in incidence is a result of increased awareness. More people know about Lyme disease and are reporting the illness.

For more information on Lyme disease, read the full Washington Post article.