Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.
Viruses are classified on the basis of morphology, chemical composition, and mode of replication.
Most viruses replicate by hijacking a cell’s DNA and forcing the cell to make more viruses. When the cell is full of viruses it bursts and releases many more viruses to infect more cells. Symptoms increase in an individual until the body’s immune system can ID and catch up with destroying the virus. The body identifies a virus by the proteins on a virus protein shell. Some viruses mutate a lot, which includes changing the protein shell, that’s why vaccines are so difficult to make for some viruses, it’s because that protein shell is constantly changing. By May of each year, months before flu season, the common flu vaccine must be finalized by taking a guess about which viruses will be the most prevalent in a region. It’s just an educated guess, sometimes the flu vaccine helps, sometimes it doesn’t, sometimes it helps some people, sometimes not others.
Scientists still cannot agree if a virus actually meets the definition of life, as the virus cannot replicate by itself, it needs a host cell to do so.
Some common examples of DNA viruses are parvovirus, papillomavirus, and herpesvirus. Examples of RNA viruses are Rotavirus, polio virus, yellow fever virus, dengue virus, hepatitis C virus, measles virus, rabies virus, influenza virus and Ebola virus.
