The Sims Tips

Guinea Pig Disease
There are three ways to cure the guinea pig disease. The first one is good old rest and relaxation. The second one is to buy the Chemistry set and to make your sims make a white potion. The odds of producing one is only one in eleven. The third and most reliable way is to purchase the Forgotten Guinea-Pig Painting.
To avoid this disease, look after your guinea-pig!

Addition from barbieg13:

the forgotten guinea pig painting is actually a way to get the disease. If you have the painting+neglected guinea pig and play with the guinea pig, it can bite you and you can get the disease.
if you don't want the disease, don't buy the painting.