WASHINGTON, D.C. (WPVI) -- Unfortunately the population of bugs that carry the disease has exploded.
They are black-legged ticks, snd they are very small - just about the size of a pin head.
But they have an enormous impact.
The number of Lyme Disease cases has tripled in the past 20 years.
A new report shows the ticks are in about half of all the counties in the U-S.
They've been established in eastern Pennsylvania, New Jersey, and Delaware for some time.
But they're now in states like Ohio, where they weren't believed to be.
Doctors hope the new Lyme disease tick map will encourage doctors in areas like Ohio to consider Lyme when a patient shows the typical flu-like symptoms.
Scientists also know now that northern ticks are more aggressive at finding hosts than southern ones,one reason why there has been such a boom in cases in the East.