NEW DELHI — Police officials in northern India were searching on Tuesday for a fake doctor suspected of infecting dozens of his patients with H.I.V. by reusing a dirty syringe.

The patients were treated by what is known as a “jhola chhaap doctor,” a wandering medical practitioner whose only verifiable qualification (a chhaap is a trademark or official seal) is a jhola, the cotton shoulder bag from which treatments are dispensed.

Most are untrained, and some of the cures they offer can be dangerous, but in India, where the health care system is tremendously challenged, many poor people often feel they have no choice but to pay a few rupees for their services and hope for the best.

In this case, health officials in Unnao, a primarily rural district two hours’ drive southwest of Lucknow, became concerned last July when an unusual number of patients visiting a government hospital began testing positive in routine H.I.V. screening.