+1 on changing that air filter. The air that you breathes makes a big difference. uncirculated fresh air from a bunch of sick people is really bad. You should leave the windows in your house open also to let fresh air flow in everynow and then also.
And what also helps big time to fight sickness is to drink alot of water and get plenty of excercise. Know its hard when you're sick but it really works. If I have the flu I do this and it's normally gone the next day
Hope you get better!