The immune system protects the body against infection. However, it's more like a little army than a giant shield. Germs, bacteria, and viruses primarily enter the body through contact with infected surfaces or through the air. The microscopic army fights off the invaders and remembers their weaknesses for next time. The next time your body encounters a similar infection, your immune system is better equipt to fight it. This process also happens on a smaller scale when you get flu shots and vaccines.
An important part of this system is white blood cells, which seek and destroy germs as soon as they enter the body. Doctors can test the levels of white blood cells in your body to see if you have the flu or a similar infection. If you have a high level of white blood cells you are probably fighting off an infection. You can boost your immune system by maintaining a healthy diet and lifestyle.