"How do I avoid getting sick?"

Hand washing
Even washing with water rinses off more than 50% of the bacteria on your hands and helps to prevent infecting others. Washing with a bacteriocidal soap and hot water for 30 seconds will wash off more than 65% of the bacteria.

Cover your mouth when you cough.
Aerosolizing bacteria and viruses are the main vehicle for transmission. Sneezing or coughing fires the bugs in a wide radius around your mouth and someone passing by can inhale these and get sick.

Don't touch you face.
Bringing bacteria and viruses to your nose and mouth with your hands is the surest way of getting sick. If you have to scratch your nose or itch your eye use the earpiece of your glasses or the inside lapel of your shirt or coat. Having Kleenex is the best since you can throw this away.

Don't share a glass or straw.
Putting a glass or straw to your mouth when someone else has just done the same thing is one of the surest ways to get sick. Fresh, warm saliva is one of the best transport mediums for bacteria and viruses.

Take two showers a day.
After sitting in class all day and being close to others who may be sick your exposed skin and hair will be coated with viruses and bacteria. If you go home and then go to sleep at night these contaminants get smeared onto your pillow case and your face has hours to make contact. If you take a shower at night before going to bed you will reduce the exposure and contact time to these germs and viruses.

Don't suck your pen!
This brings bacteria and viruses to your mouth directly.

"Is there a pill I can take to make it so I don't get sick?"
No. Get enough rest, eat well, don't smoke cigarettes, exercise. Looking out for your health is a series of daily steps to take care of your immune system and allowing your natural defenses to fight diseases you might be exposed to.