Description of how small viruses are, and how infectious they can be. Viruses have one or more special surface proteins with which they attach to human cells and gain entry. HIV, for example, can bind to three different cell surface proteins on human T cells. An explanation that viruses arose from incompletely synthesized bacteria, which occasionally survived as small sacs containing some RNA that coded for the necessary proteins to make new viruses. Viruses are duplicated with the aid of human enzymes. Being generally inside human cells, it is difficult to develop medications that will not also harm the human cell. Common flu viruses have two surface proteins: H (hemagglutinin) and N (neuraminidase), which are recognized by our antibodies. Flu viruses have a very sloppy polymerase, so that viral proteins mutate frequently; there are now 18 types of H proteins and 11 types of N proteins, so that more than 130 variants of these two combinations have been observed. That is why we need new flu vaccines about every 2 years. The virus causing HIV-AIDS infects our T lymphocyte cells, and therefore people do not die from the virus itself, but from various infections that occur after the immune system ceases to function. A subset of people does not have a complete CCR5 receptor, making them immune to viral infection. It is thought that this mutation emerged during the epidemic called the black death plague in the fourteenth century. The virus spreads by having its RNA reverse transcribed to DNA by the viral reverse transcriptase, and then becoming inserted into human DNA, where it will remain. The SARS Covid virus causes severe acute respiratory symptoms and has a surface spike protein that binds to the ACE2 enzyme on epithelial cells. Therefore, it can reach and infect every cell in the body. Modeling of this spike protein has identified numerous “hot spots” where future mutations could occur and make this protein no longer recognizable by current vaccine antibodies. Human deaths from Covid exceed mortality numbers for all wars.