A microorganism (so small, it can only be seen under an electron microscope) that relies on another living organism or host cell to grow or reproduce.
Viruses invade or infect other living cells and hijack them to reproduce themselves. They can infect a variety of living host organisms, including bacteria, plants, animals, and humans. While most viruses are harmless to humans, some can make you sick and some can even be deadly. Illnesses caused by viruses include influenza, chickenpox, measles, AIDS, rabies, the common cold, and COVID-19. Antibiotics are not effective against viruses but vaccines and antiviral medication can be.
A virus consists of nucleic acids DNA or RNA enclosed in a protein coat. When a virus enters or infects a cell it sheds its coat, and induces the cell’s own replication machinery to reproduce the virus’s DNA or RNA instead. By hijacking the cells replication machinery the cell manufactures more viral protein based on the instructions in the viral nucleic acid. The newly created viral fragments assemble, and more virus is made which can then infect other cells.
TIP: Do not confuse viruses with bacteria and vice versa. They are different kinds of microorganisms and cannot be used interchangeably.