When Rimmer said that he was going to decontaminate the old Officer's Quarters, Lister explored them before Rimmer had actually done so. Lister caught the disease whilst in there, and Rimmer hypothesized that is was a three million year-old, mutated form of pneumonia, although it is unclear if this is true.
The disease had a bizarre side-effect - it could manifest Lister's hallucinations into reality, resulting in an exploding Mayor of Warsaw, raining fish, and the appearance of Lister's Confidence and Paranoia.
Confidence killed Paranoia by feeding him into the Waste Grinder, so that he could be alone with Lister. Confidence also destroyed the Medi-Bot so that Lister couldn't be cured of his space pneumonia, ensuring Confidence's lasting existence, since Confidence knew that he was a symptom of Lister's illness. However Confidence unwittingly killed himself when, during a space walk, Confidence removed his helmet to prove that "Oxygen is for losers".