One by one, all the female members of the Legion of Super-Heroes fall ill and collapse. Even Night Girl, who goes to help them, succumbs to the same malady. Mon-El announces that they all have a mysterious “Crimson Virus” and that there is no known cure1. Worried that the infection may spread, the male legionnaires ship all their female members off to “Quarantine World” — but not before doing their best to cheer them up:
Saturn Girl: Must we go into exile, Superboy?
Superboy: I’m sorry Saturn Girl, but all of the female members of the Legion are doomed. You must go to Quarantine World! Let’s hope we can find a cure before you die!
A short time later, a masked woman calling herself Satan Girl shows up and offers to join the Legion. When she is turned down, she announces that she is the one who made all the women sick, and now — out of spite — she’s going to kill them. She heads off to Quarantine World to finish the job. Luckily, Supergirl arrives at that moment, discovers what has been happening, and heads off to stop Satan Girl. She arrives just in time and a vicious battle erupts. Surprisingly, Satan Girl is am equal match for Super Girl and manages to escape, but not before swearing to return:
Satan Girl: You’re clever, Supergirl. But I’m just as clever as you — and just as strong! I’ll be back later and I’ll bring doom for you and all your precious Legionnaires!2
The sick Legionnaires are moved to another planet3 and then another to hide them from Satan Girl, but she always manages to find them. Supergirl sets traps for her nemesis, but they are always avoided or ineffectual. In the end, Satan Girl manages to defeat Supergirl by burying her under green kryptonite.
Overconfident, she turns on the male Legionnaires but is defeated by the Legion of Super-Pets4 — and then the truth is revealed. Satan Girl is really an evil double of Supergirl that split off when Supergirl unknowing collided with a red kryptonite meteor. The evil double knew she only had 48 hours to live unless she could somehow rid herself of the red kryptonite radiation. She was able to project the radiation into the female super-heroes, causing their illness. But now her 48 hours are up and she fades away to be absorbed by the original Supergirl. With Satan Girl gone, the “Crimson Virus” vanishes and all the Legionnaires are instantaneously cured5.
Notes:
1. There is never any actual virus in this storyline, so why is Mon-el so adamant that the disease was caused by the Crimson Virus? In fact, it turns out not to be an infectious disease at all.
2. It would have been a better story if she had brought Doom, and then they played video games all night long. But then Jack Thompson would have blamed them for the rising violence among Khund children and it would have all gone downhill from there.
3. One of the planets seems to be inhabited entirely by Madballs.
4. That would be Krypto (the Super-Dog), Streaky (the Super-Cat), Comet (the Super-Horse), and Beppo (the Super-Chimp).
5. Despite being 1000 years in the future, the Legionnaires sure have some dismal medical care. Sure they have fourth dimensional surgery, but when you get down to it, that’s actually pretty backwards. An earlier issue showed that they keep a powerful healing urn stored in a museum — why not actually use it? This is what happens when you have a Coluan who thinks that he’s a doctor.
Source: Adventure Comics #313, “The Condemned Legionnaires,” by Edmond Hamilton and Curt Swan

Other colorful comic book diseases:
Black Flu,
Green Plague (1),
Green Plague (2),
Purple Plague,
Red Rain, and Scarlet Jungle Fever.