In the sims2 seasons as you know you will get sick sometimes, usually because of staying outside in the cold for too long. If you are sick, you should quickly go inside, eat or make sure you are have full hunger, and then sleep. Also maintain all needs with green level. After continue to rest, and later on a pop-up will say your sim is healthy. Also while you are still sick avoid working, going outside, and other sick sims! And do not eat any spoiled food, you will know it is spoiled if it is green with flies around it. This should cure your sim!
I hope this helped you to answer your question!
HAPPY SIMMING!!! : )