The Michigan jail system is fighting the spread of hepatitis A in the state’s three largest counties.

The Detroit News reports that thousands of jail inmates in Wayne, Oakland and Macomb counties have opted to receive the hepatitis A vaccine for free in recent months.

A spokeswoman for the Michigan Department of Health and Human Services says the vaccine costs the state $25 per dose.

Health officials say those in jail are particularly vulnerable to hepatitis A because of the close quarters inmates share.

The Health Department says southeast Michigan has had 715 cases of hepatitis A and more than 20 deaths since August 2016. Health officials say the majority of cases are among drug users, homeless people, and current and former inmates.