Ocular syphilis, a sexually transmitted infection once so rare people forgot it existed, is back. The disease can send people blind if not treated quickly, and lack of awareness is making that much more common.

When generations of horny adolescents were warned to “stop it or you'll go blind”, the reference usually was to a safe form of sexual pleasure as there was a real way sex could lead to loss of vision. Ocular syphilis occurs when Treponema pallidum, the bacterium that causes syphilis, infects the eyeball, causing inflammation of one or more tissues.

As antibiotics appeared to sweep all forms of syphilis into history's trash can, the condition disappeared from doctors' view. Now, however, it's back, with cases surging around the world. Professor Justine Smith of Flinders University conducted a study of 127 cases from Brazil, a global hotspot for ocular syphilis, to try to work out what is causing the revival and what we can do to stop it.

In Scientific Reports, Smith observed 68 percent of her sample were infected in both eyes. Most cases affected the retina, causing blurriness and “floaters”, rather than the soreness and red eyes associated with inflammation of the front of the eye, although these symptoms sometimes occur. Half were too affected to drive safely.

A third of study participants tested positive for HIV. Smith told IFLScience that being HIV positive increases susceptibility to all forms of syphilis, and rates of ocular syphilis rise proportionally, rather than there being something specific about the paired infections that triggers eye inflammation. Although the reasons for the relationship are not fully understood, Smith said: “HIV affects the immune system, and the treatments also affect the immune system.” It appears these increase susceptibility to syphilis.

Meanwhile, condom usage has fallen, at least in some communities, as fear of AIDS declines, leading to spikes in many sexually transmitted infections. Unlike gonorrhea, whose rise is associated with antibiotic resistance, Smith told IFLScience penicillin remains effective against syphilis, and there are other effective antibiotics for those who are allergic.

However, the lack of obvious symptoms mean that many people don't realize they have a problem until the infection is advanced. Additionally, the long period when ocular syphilis was almost unheard of has meant many doctors don't recognize the condition when they see it.

This is disastrous because early treatment can reverse the effects to the point where many patients at least partially recover their vision. On the other hand, Smith said in a statement: “When ocular syphilis goes untreated or is treated late, the damage done to internal components inside the eye may be permanent.”

Consequently, Smith argues, the priority is to alert doctors to be on the lookout.