I rode in the rain yesterday. This will be my third season as a foul weather bike commuter. Here are my tips:

1) Light yourself up like a Christmas tree. BE SEEN. Drivers aren't expecting you to be out there on days like this.
2) Come down hills slowly, and approach turns at the bottoms of hills VERY cautiously, especially if you have to cross painted lines in doing so.
3) It's true what they say about painted lines. VERY slippery. I almost went down crossing one on a turn. Best not to *****foot around the lines either, I managed to correct the slip and avoid going down by maintaining adequate speed as I crossed the line (line was about 12" wide). The longer you stay on the line, the better chance of you slipping.
4) I don't know where you are riding to, or if you are just looping back and coming home. But if you are going somewhere like school or work, make sure you have ALL the spare clothes you need, including a 2nd set of biking clothes. And is your backpack waterproof? I found out yesterday that my seat bag is NOT waterproof. You will be soaked on your way there, do you want to put on wet clothes before coming back? Make sure your spare clothes are properly protected from water. I got lucky yesterday because I happened to have a full 2nd kit in my locker at work.
5) Consider putting fenders on your bike, especially if you plan to continue riding in the rain. They won't keep you from getting wet, but will help you avoid getting a stripe of dirty water up your back and front sides (spray from tires). And they will help keep your drivetrain somewhat cleaner.
6) Will you get sick? Depends on you. Put a beanie on under your helmet, take your vitamins, and pray!