As it turns out, syphilis is a STD that can have disastrous consequences for your eyes.

This Thursday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report (MMWR), doctors reported on a cluster of ocular syphilis cases across the Northwest and California that occurred this past winter — cases which, for some, tragically ended in blindness.

Syphilis, more politely known as the spiral-shaped bacterium Treponema pallidum, is a far-reaching disease that manifests discreetly as a vague collection of symptoms; from an ingrown hair-like bump in the beginning to skin rashes in the early-to-middle stages of infection, and lastly to a vast assortment of neurological and physical symptoms, including dementia. Before that often gruesome finale, a person can harbor syphilis with little to no signs of it for decades.

In this instance, syphilis decided to take hold in the eyes of four King County, Washington men, causing various degrees of vision loss and inflammation — with at least one complaint of seeing flashing lights. All were men who had sex with other men, and three had a concurrent HIV infection. After treatment with antibiotics, they improved, but five months later, two were declared legally blind. Given how close together these cases happened (all between December 2014 and January 2015), an advisory was sent out to medical providers in the greater West Coast area.

A subsequent investigation by the San Francisco Department of Public Health located eight cases of ocular syphilis between December and March 2015. The sufferers were primarily men (seven cases), in particular men who have sex with men (six cases), and HIV-infected (seven cases). The lone female case involved a sex worker. As before, there was noticeable visual improvement after treatment, but one patient suffered permanent vision loss in one eye after three months.

Though syphilis remains readily curable with penicillin, it’s believed that cases involving the eye can be more difficult to treat because it’s harder for antibiotics to quickly reach eye tissue. Syphilis of the eye is likely to be a harbinger of worse tidings since it indicates that the bacterial germ has found a comfy home inside the brain and nervous system, but most of the time, as with the patients seen here, it occurs in the early stages of the disease.

It’s also exceedingly rare, with some research showing that it occurs in 2.5 percent of all syphilis cases. Still, the CDC authors advise that physicians remain on the lookout for visual symptoms when treating syphilis patients, and emphasize that any ocular syphilis cases need to be reported to the local or state health department within 24 hours of discovery.

Source: Woolston S, Cohen S, Fanfair R, et al. Notes from the Field: A Cluster of Ocular Syphilis Cases — Seattle, Washington, and San Francisco, California, 2014–2015. Morbidity and Mortality Weekly Report. 2015.