There is reason for urgency. The transmission rate has been about 80 percent, Dr. Cooke said, meaning that eight in 10 of those who have acknowledged sharing needles with someone who has the virus have tested positive. That indicates “a very high viral load in the community right now,” he said.

Health officials from nearby Clark County, which handles H.I.V. testing in 11 counties, have been going out since February to test people at their homes, using oral swabs that deliver preliminary results in 20 minutes and asking those who are tested to disclose any needle-sharing and sexual partners.

For the last week, a team from the C.D.C. has been helping with that work; a spokeswoman for the agency said it was not aware of any other jurisdiction’s ever declaring a public health emergency because of an H.I.V. outbreak.

For those who are H.I.V. positive, case managers have been assigned on the spot to help arrange counseling and treatment. But most have not started treatment yet, partly because the closest H.I.V. clinic is in Louisville, Ky., about 35 miles south of here.

“Our first three, we had appointments for them at the Louisville clinic, and to a person, none of them showed up,” said Dr. Kevin Burke, the public health officer who oversees H.I.V. testing in the region. “That may be fatalism. But at the same time, this population doesn’t have reliable transportation.”

Some will not be coming to the clinic for now because they are in the Scott County jail. Sheriff Dan McClain said that 11 of his 120 inmates had tested positive so far, but that they had not started treatment because the cost is so high — upward of $20,000 a year per patient, according to Dr. Burke — and he wants a guarantee of financing first.