Viruses

microscopic · infectious · biology

Invisible invaders. Viruses are so small you can't see them without a powerful microscope, but they're the most common living thing on the planet. They infect animals, plants, bacteria, and every other kind of organism.

Need a host cell. Unlike bacteria that can live on their own, viruses can only survive and make copies of themselves inside the cells of other living things. Once inside, they hijack the cell's machinery to reproduce.

Everywhere on Earth. Viruses live in almost every ecosystem, from ocean depths to soil to the air we breathe. Scientists have described more than 16,000 different types, but there may be millions more waiting to be discovered.

Recently discovered. People only learned viruses existed about 130 years ago when scientists found that tobacco plants were being infected by something smaller than bacteria. Before that, no one knew viruses were even real.