Viruses make a person sick by invading the living cells in the body and use them to grow and multiply. With time a person's immune system becomes weak, thus making him or her feel sick.
Viruses are small organisms, much smaller than bacteria, that cannot do much on their own. Therefore, they need a host that provides them everything they need. Conditions caused by viruses can be hard to treat because the viruses live inside the body's cells. This, in turn, protects them from medications intended to kill them. Antibiotics do not work for viral infections, as stated by MedlinePlus. Examples of viral infections include HIV/AIDS, flu, common cold, warts, small pox among others.