In retrospect, Jennifer Mankoff, now 37, believes she was infected with Lyme disease either during a trip to Ligonie, Pa., in 2005 or while hiking in Frick Park in Pittsburgh in the fall of 2006.

She got a rash, one whose cause was never diagnosed, after the Ligonier trip, and she actually picked a tick off her leg after the hike in Frick Park.

Either way, the Shadyside, Pa., woman, an associate professor in Carnegie Mellon University's Human-Computer Interaction Institute, got sick later in 2006. She's not exactly sure when the symptoms started, but she was so ill that she had friends come stay with her when her husband had to travel in December.

Lyme disease is caused by a bacteria carried by ticks most commonly referred to as deer ticks, although entomologists now identify them as blacklegged ticks. They have been infected as larvae and nymphs, which feed on birds or small mammals. Adult ticks prefer deer. Any stage can feed on humans, potentially passing on the disease. It is the most common tick-borne illness in North America and Europe, and, says Lyme disease researcher Andrew J. Nowalk of Children's Hospital of Pittsburgh, it is a "devastating disease for patients who have it."

The infection, once located primarily in New England and the mid-Atlantic and north-central states, is moving westward through Pennsylvania.

In 2009, according to a state-by-state report issued by the Centers for Disease Control and Prevention, Pennsylvania had 4,950 confirmed cases plus 772 probable ones. That's up from 3,985 cases in 2004.

But those numbers are deceptive, say both Stephen Ostroff, director of the Pennsylvania Bureau of Epidemiology and acting state physician general, and Kevin Griffith, medical epidemiologist officer in the CDC's Division of Vector-Borne Diseases.

There are several reasons for the statistics besides a simple spread of the Lyme bacteria. They include a change in the CDC reporting procedure that has led to increased surveillance, Griffith said; what Ostroff calls "a greater recognition and appreciation" that has led to more testing and identification of Lyme; and an increase of people moving into previously wooded, high-risk areas.

The CDC also has recently changed the definition of a positive Western blot, the test used to confirm the presence of Lyme antibodies in the blood, which also could lead to more reported cases.

Nevertheless, Ostroff sees a geographic spread of the Lyme bacteria.

"When you look at it nationally, the numbers are increasing, and we also see ... a westward movement,"he said.

Similarly, Nowalk cites both greater awareness of Lyme and a spread of the Lyme bacteria as reasons for an increase of two to three times the number of cases — he estimates a total of 60 or 70 — seen by Children's Hospital last year. He also said experts believe the "true number"of Lyme cases is two to three times greater than that reported.