Viruses are capsules with genetic material inside. They are much smaller than bacteria. They enter cells of the body and use the cells to produce more viruses eventually killing the cells. The cell death and the body's immune response to fight the virus cause the symptoms experienced. Viruses cause a whole host of diseases that range from the common cold and flu to AIDS and hemorrhagic fever. Viral infections do not respond to antibiotics. There are only a few anti-viral medications available. Vaccines can prevent some viral infections.