A virus is a germ that is smaller than a bacteria. Viruses are encapsulated by a protective coating so they are more difficult to kill than bacteria. They are very tiny, much smaller than bacteria. Viruses cause familiar infectious diseases such as common cold, flu and warts. They also cause severe illnesses such as HIV/AIDS, smallpox and hemorrhagic fevers. Antibiotics do not work for viral infections. Vaccines can help prevent you from getting many viral diseases.