A virus is a very small nonliving thing that invades the cells of living hosts. Viruses can come in many different shapes but the structure of most viruses is pretty standard. All viruses contain their genetic information encased in a protein outer case. Viruses work like a lock and key with their host cell. Viruses will only attach to and infect certain cells in which their outer proteins match up. This is why certain viruses you contract only affect certain parts of your body. Although viruses are non-living they are able to multiply and make copies of themselves once they have found a host cell. Viruses can cause many types of sicknesses from the common cold to the AIDS virus. In addition humans are not the only organisms affected by viruses. Viruses can affect all tiny single celled organisms, plants and animals. Although we generally associate viruses with bad things they do have some positive uses. They are being used in gene therapy and are used to deliver genetic materials to cells in hopes to find treatments for several diseases.