File: A man uses heroin under a bridge where he lives with other addicts in Philadelphia in 2018. In San Diego County, the use of black tar heroin has been linked to flesh-eating bacterial infections. (Spencer Platt/Getty Images)

Seven people in San Diego County have died since October due to a flesh-eating bacterial infection associated with black tar heroin.

The bacteria produces a toxin which can rapidly break down soft tissue. It's a rare condition called myonecrosis. Symptoms include severe pain and blisters. If not treated, it can lead to amputation and death.

"This is the first cluster of cases that we've had of this myonecrosis in the 10 years I've been here," said Dr. Eric McDonald, medical director with the epidemiology program for the County of San Diego's Health and Human Services agency. He says seeing this many people coming down with it at the same time is unusual.

McDonald notes that cooking heroin can kill many bacteria but it doesn't eliminate the spores that cause these illnesses.

So far L.A. County hasn't seen any new cases of myonecrosis, but health officials are monitoring two confirmed instances of a similarly transmitted illness — also linked with black tar heroin — called wound botulism.

"We have not seen any deaths like San Diego," said Dr. Dawn Terashita, associate director of the acute communicable disease control program for L.A. County. "But we have seen an increase in [wound botulism] cases, so we are worried about this and are working on it."

Terashita said botulism spores thrive in low-oxygen conditions and that's how black tar heroin is produced.

L.A. County Health officials are working to get the word out to drug users to warn them that the dangers of using heroin go beyond overdose.

"We have started a campaign to target high-risk clinics or high risk areas where these patients may be seen," Terashita said.

According to the California Department of Public Health, local health agencies have been "advised to consider reaching out to rehabilitation centers, shelters, clinics, and needle exchange programs to alert drug users of the risk of severe life-threatening skin and soft tissue infections from injecting black tar heroin."