The number of illnesses that are caused by viruses are almost uncountable. They have been responsible for the deaths of millions, from smallpox to the Spanish flu was responsible for more deaths than were direct casualties in World War I to polio. Even in the 21st century, the flu can be fatal for hundreds of thousands of people every year. Trying to find ways to combat viral diseases is a never ending challenge for the medical community.
Information. Even today, people don’t quite know what a virus is. It is not quite alive but it is not inanimate. Some viruses are made up of only a strand of genetic material inside of a protein coat while others are more complex. They are much smaller than bacteria and cannot be seen with a regular microscope. They can only reproduce inside the cells of living beings, including humans and all other animals and plants. There are over 300,000 viruses that affect mammals alone. Viruses even attack bacteria and single celled life forms known as archaea.