Illness

With the infamous guinea pig disease a thing of Sims 1 past, Sims can now catch a few different diseases, each with there own symptom and method of catching- they're contagious, too.

Morning sickness
Not strictly an illness as it can't be cured and only lasts one day. The first in-game sign that a Sim is pregnant. Throughout the day the Sim in question will run to the toilet to throw up, or gesture 'throwing up'. A thought bubble is most likely to appear with the pacifier and a question mark, as if the Sim was thinking if they are pregnant.

Food poisoning
There is a 5% chance any time a Sim eats spoiled or burnt food that the Sim will get food poisoning. Sims will suffer nausea (a sharp drop in comfort) diarrhea(a sharp drop in bladder) and will run to the toilet at regular intervals.

Cold
A 5% chance when standing next to or near Cockroaches or any time a Sim goes to work, school etc. Pretty obviously Sims will suffer from coughing and sneezing fits causing a drop in comfort.

Flu
If the cold is not cured in three days it can become the flu. The flu has all the symptoms of a cold with sharp drops in fun and energy as well.

Pneumonia
If the flu is not cured quickly it can turn into pneumonia. The pneumonia has all the symptoms of both the cold and the flu, as well as sharp and sudden drops in energy in which a Sim appears to nearly pass out. If energy drops low enough, the Sim really will pass out.

Mysterious disease
Can be infected by when using the biotech station. The symptoms are random and can be any of the above.

Curing
The best cure for all illnesses is rest. Keep the ill Sim away from healthy Sims, making them relax for the period of their illness until a message appears saying they are cured.

Passing it on
Not all illnesses are contagious and those that are become contagious at different rates. As food poisoning is an infection from only one source and morning sickness only affects pregnant Sims these are not contagious. The mysterious disease is more contagious than the flu and the flu is more contagious than the cold.

Death
If the illness is not treated within ten Sim days, the Sim will die from illness (this can also happen to children).