Light virus
The light virus was a pathogenic virus found on Hetemit IX. It infected humanoids and passed from person to person in the form of additional energy on electromagnetic waves. The virus activated the brain's pleasure centers in response to light; those afflicted exhibited a critical drop in cholecalciferol and a compulsive need for light exposure, to the extent that they completely disregarded their own safety.

The Illyrian colony on Hetemit IX was wiped out by the light virus. Some of the infected colonists ran into the ion storms that frequently swept the planet, where they were transformed into plasma beings.

In 2259, members of a  away team investigating the abandoned Illyrian settlement were infected by the light virus and brought it onto the ship; it was missed by the transporter biofilter because the medical transporter had not been upgraded to separate the biofilter and pattern integrity power systems at Doctor Joseph M'Benga's insistence. The disease quickly spread among the crew, despite the implementation of a level 5 lockdown protocol. Only Number One remained unaffected, as her enhanced Illyrian immune system had purged the virus from her body. However, this process left no physical traces on which to base a treatment.

Under the influence of the virus, Lieutenant La'an Noonien-Singh deactivated the containment field around the warp core. The rising radiation levels within main engineering activated Number One's immune system, which induced the production of chimeric antibodies against the light virus within Noonien-Singh's system. Before the antibodies set, Nurse Christine Chapel was able to synthesize them into a cure for the rest of the crew. Lieutenant Spock would later speculate that, had the Illyrian colonists not been attempting to reverse their genetic modifications in the pursuit of Federation membership, they would not have met their fate. ()

Microorganisms
Medical conditions