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CHICAGO — The health department says a small but serious outbreak of invasive meningococcal disease appears to be affecting sexually active gay men, so they are being urged to get vaccinated.

The Chicago Department of Health urgently pulled together Wednesday’s press conference after determining Tuesday the 3 reported cases that have been reported in the past three weeks have all come from the same strain.

They have also traced the outbreak to a specific group of people: sexually active men who are having sex with men living with HIV, or sexually active men who are having sex with anonymous partners or who are using online hook-up apps to meet other men.

The invasive meningococcal disease is not as contagious as he common cold or the flu, but it is spread thru close intimate contact with saliva, which includes kissing, sharing drinks, even a cigarette.

It also can be spread through sex. Health officials say the disease is a rare bacterial infection that can cause very serious diseases, such as meningitis or blood stream infections and if left untreated it can lead to death.

The symptoms: fever, headache and a stiff neck. Some other symptoms include nausea, vomiting, increased sensitivity to light and confusion.