For all the gory details you can look at The Plague: A Detailed Study at MATY. The one correction I'll make is each cycle is not an hour. There is another delay buried in the code that makes each cycle 60 minutes PLUS 0-60 minutes per disease token, or approximately 1 hour plus 1/2 hour per sick sim. Every 6 cycles it updates severity.

Overall it's interesting and complex system...that ultimately doesn't really do much. I'll give the short version here.

Each disease starts with a level of severity. Severity changes over time based on how much the sim rests. If the severity reaches the lethal level, which is based on the disease type, the sim dies. If severity reaches 0, the sim is cured.

There are two exceptions. Colds don't kill sims but transform to pneumonia if they reach the "lethal" level. Morning sickness doesn't change in severity and can be neither lethal nor cured early.

Under the unmodified system, high recovery rates make it nearly impossible to die of disease even if you try.

So how do you get sick? I've found the following ways, but I suspect I'm missing at least one disease vector. Twice I've had sims abruptly get sick when none of the situations listed below applied.

  • Spoiled Food - When taking the first bite of spoiled food, there is a 10% chance of contracting food poisoning
  • Working - Each day when returning home from work there is a 1% chance of contracting a disease. This will be either a cold or the flu, 50% chance of each.
  • Biotech Station - When making medicine there is a 0% to 65% chance the sim will instead catch the mystery disease. The chance goes down with high Logic, Mood, Nice and Hygiene.
  • Roaches - Every 4 hours roaches broadcast the flu to all sims within 2 tiles. Exterminators and evil witches are immune to roach disease
  • Contagion - Sick sims leave invisible plague tiles every time they express symptoms such as a cough or sudden bladder drop. These tile last for (20 * contagiousness) seconds so range from a bit under 7 minutes for pneumonia to over 30 minutes for the most contagious versions of the mystery disease. Every 50 seconds these tiles infect anyone standing on top of them.
  • Pregnancy - During the first 25 hours of pregnancy, meaning before the first baby bump, male and female sims will suffer from morning sickness.

Note that non-playable sims (townies, NPCs, tourists, locals, etc.) are supposed to be immune to disease as are roommates and ghosts. I'm not sure that's checked consistently because I know I've had townies get sick.