Washington State health officials have reported a sudden cluster of rare syphilis infections of the eyes—detected in six people, two of whom have gone blind, The Seattle Times reports. Ocular syphilis typically strikes fewer than one in 1 million Americans in an entire year; these cases have all been reported since mid-December. Just 10 percent of all syphilis cases involve ocular disease.

Four of those infected live in King County, where Seattle is located. Three of them are men who have sex with men (MSM), and three are HIV-positive.

All six of those who contracted the infection reported vision problems, including vision loss, blurriness, developing a blue tinge in their vision, and seeing “floaters” and flashing lights.

The cause of the cluster of infections is unclear.

To read the Seattle Times story, click here.