Virus
Viruses can only be seen under an electron microscope. Viruses are not free-living: they can only be parasites. They always reproduce inside other living things. All viruses infect living organisms, and may cause disease. The virus make copies of itself inside another organism's cells. Viruses are a strand of nucleic acid with a protein coat. Usually the nucleic acid is RNA; sometimes it is DNA. Viruses cause many types of diseases, such as polio, ebola and hepatitis.