They are too small to be seen through a light microscope and can be visualized only by an electron microscope. Viruses are not considered to be living organisms because they cannot reproduce outside of a living cell. A virus binds to a cell when a part of the outer viral structure docks with a specific molecule on the cell surface. Viruses multiply by inserting their genetic information into a cell and harnessing the cell machinery to replicate the viral genetic material and make new capsids. The new viruses are then transmitted to other cells, either through the cell wall or when the cells burst and die.
Viruses are parasites on a cellular level.
New strains of disease causing viruses are a constant threat because viruses mutate easily. Each year, a new influenza virus causes illness in millions of people. As a result and because most drugs cannot disable a virus without harming healthy tissue, it is difficult to develop drug therapies to cure viral infections. Viruses can be transmitted from one animal species to another. They affect each species differently and an animal host can harbor viruses deadly to man such as the Ebola and Hanta viruses, without any ill effect.