Choriocytosis is a virus affecting the ability of blood cells to carry oxygen to the body's organs. It is known to affect Humans and Vulcans.

While Humans are minimally affected, the virus is extremely deadly to Vulcans with their copper based blood system; death can occur within a week as the victim essentially suffocates internally. Initial symptoms include increasing lethargy as the body loses access to oxygen.

The only known treatment is strobolin, a rare substance that is only found on a few planets. A synthesized version can be administered to slow the disease's progression, but the recipient quickly becomes immune within hours. Obviously, this kind of infection is considered a medical emergency, prompting coordinated delivery with multiple ships to deliver the medication in a timely manner.

In 2270 the USS Enterprise was affected by an outbreak of choriocytosis that was eventually brought under control by the administration of strobolin. (TAS: "The Pirates of Orion")