There are many different types of viruses. Some viruses cause diseases, and some do not. Viruses are all very small, and they are all made out of proteins and genes. Some viruses also have a membrane, like cells do. There is a debate about whether viruses are alive or not. They may be alive because they are able to copy themselves, and they are made of protein and nucleic acids, like all other life is; however, viruses cannot do anything by themselves. Because they are completely dependent on a host for everything, some people do not consider them to be alive.
In order to copy themselves, viruses must enter a cell. Once they are inside, they take over the machinery in the cell and use it to copy their DNA or RNA and to make virus proteins instead of the normal proteins a cell would make. Different kinds of viruses can grow in just about any kind of life. There are viruses that infect animals, plants, and even bacteria. Since viruses are so good at taking over cells, they make loads of virus copies that then go on to infect other cells. There are some viruses that make so many copies that there are a trillion (1,000,000,000) viruses in one milliliter of blood (about the size of a sugar cube). This is five times more viruses in a few drops of blood than there are stars in the galaxy! When viruses take over a cell, they kill it or disrupt it. This is what makes you sick. Your immune system fights the virus, and many of the things it does make you feel sick also. Your body gets fevers because sometimes viruses do not grow as well at higher temperatures. Because the viruses are inside your cells, your immune system has to kill the infected cells before they can make more viruses to prevent the virus from multiplying.
Viruses actually do not make us sicker than bacteria. In fact, you never even know you have many viral infections. We do not have as many problems with bacteria as we used to, because bacteria can usually be treated with antibiotics. Except in a few cases, such as HIV and herpes-viruses, there are no drugs to treat viral infections. We may feel worse with a viral infection than with a bacterial infection, because our only choice is to suffer through it. In major infections, however, bacteria are generally much more dangerous than viruses. There are bacteria that make the most deadly poisons on earth. Also, there are bacteria that are resistant to all known forms of antibiotics. There are only a very few viral diseases that are deadly, such as smallpox, Ebola, and Hantavirus. The biggest danger to viral infections is generally that they will weaken a person to the point that they cannot fight off bacterial infections.
Viruses are fascinating, teetering on the edge of living and nonliving, unable to do anything by themselves but able to completely take over cells they invade. Studying viruses has given us insights into how our own cells work. Because viruses can enter cells, they can potentially be used in gene therapy. There is even at least one virus that slows down cancer. Viruses can be dangerous, but exciting, tiny windows into our own bodies.
Fundamental Virology, third edition. ed. B Fields, D Knipe and P Howley. Lippincott-Raven. Philadelphia 1996.