What exactly are viruses?
Viruses are a strange form of (probably) life. They are not made of cells, rather they are bits of genetic material (DNA or RNA) packed inside capsules made of protein molecules. All other forms of known life are cellular in nature, so viruses are unique.
Many viruses cause disease, such as some colds, the flu, chickenpox, measles, mumps, rubella, polio, rabies, hepatitis, yellow fever. Also, HIV is a virus and causes AIDS. Antibiotics are not effective against viral infections. (When you are sick, ask your doctor if he/she thinks the illness is viral or bacterial before requesting/accepting antibiotics.)
Vaccines can be effective in training our immune system to recognize and fight viruses. Viruses with DNA are more stable and thus change less over time as they reproduce. These viruses are easier to protect against through vaccines. Viruses with RNA (a single strand of genetic material) change much more rapidly through mutation as they reproduce and vaccines are not very effective against these viruses due to their rapid rate of change. This is why a new flu vaccine is needed every year and why its not completely effective.
Are Viruses Alive?
There is debate in the scientific community about whether viruses are “alive”. See this IFOD about the difficulty of defining “being alive”: What Does it Mean For Something to be Alive? Viruses are discrete entities, reproduce and evolve, suggesting that they are alive. On the other hand, they don’t breathe, they don’t eat and are parasites that can only reproduce with help from a host cell. Here’s an article from Scientific American discussing in depth whether viruses are “alive.”
Virus Parasitic Reproduction
Virus reproduction is interesting. Viruses cannot reproduce outside of a cellular host. After a virus enters a cell, it sheds its protein capsule and causes the host cell to reproduce the virus’ genetic material and thus create more viruses, at which point the cellular host bursts open and releases the new viruses and its own debris into the environment.
Viruses’ Role in Evolution
Of course, viruses cause a lot of disease and kill a lot of microbes. See this IFOD about bacteriophages: The Deadliest Thing on Our Planet. But, viruses have played an important role in evolution. According to biologist Dr. Olivia Judson, viruses are “important sources of new genes. The reason is that as viruses move in and out of host cells, they sometimes take a few host genes with them, or leave some of their own behind. Thus, although viruses are among the most destructive forces of nature, they are also among the most potent forces of creation.” According to Discover Magazine, ” Half of all human DNA originally came from viruses, which infected and embedded themselves in our ancestors’ egg and sperm cells.”
Further, it is possible that all cellular life is descended from viruses. From Discover Magazine: “Scientists suspect that a large DNA-based virus took up residence inside a bacterial cell more than a billion years ago to create the first cell nucleus. If so, then we are all descended from viruses.”