Viruses have always been one of our greatest enemies, yet they represent one of the most primitive examples of life.

A virus is a small infectious agent that can only replicate inside the cells of another organism. Viruses infect all types of organisms, from animals and plants to bacteria and archaea.

Viruses are found in almost every ecosystem on Earth and these minute structures are the most abundant type of biological entity.

The science which studies viruses is called virology and is sub-specialty of microbiology. Here are photos made with microscopes of some of the most known viruses.

Svine flu virus

E-coli

Smallpox virus

SARS virus

HIV Virus