As the number of Lyme disease diagnoses climbs year by year, it’s also expanding its geographic footprint.

Though widely thought of as limited to the eastern United States, the disease has been reported in all states except Hawaii – and in the Western states, it’s most prevalent in California , the Centers for Disease Control says.

Still, California accounts for just a small fraction of U.S. cases. In 2023, there were more than 89,000 Lyme cases nationwide. New York had 22,000, Pennsylvania 16,000 and California only 109. And of those, about 30% were thought to have been contracted outside California, the state Department of Public Health says.

Here’s what to know about Lyme and how it spreads.

What is Lyme disease?

It’s a bacterial disease that in its initial stages presents as f

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