Viruses cause problems when they enter the body and begin to grow and reproduce. They can have byproducts that are harmful. Strep throat for example gives off substances that cause inflammation in the throat. Some viruses can cause serious issues such as nerve damage, like polio. So the immune system works to recognize and deal with cells that might cause harm.
The first time that a particular virus moves in and attacks the body, the immune system - and the white blood cells - might be slow to recognize the issue and take some time to swing into action. But the next time that particular germs shows up, the body is ready for it and reacts much more quickly. The body us designed this way so that harm by invading germs can be halted or minimized.
