Viruses are very small, microscopic particles that contain either DNA or RNA as their genetic material. They are encased in a protein coat called a capsid. Viruses come in different shapes, like spherical and rod-shaped, and some have an envelope derived from the host cell. Unlike most other living organisms, viruses do not have ribosomes or mitochondria. Hence, they cannot make their own proteins or any of the other molecules that are fundamental to life. They are genetic parasites that can only live inside a host cell and replicate inside it.
Upon entering a cell, the virus hijacks its machinery and begins to reproduce. It does this very quickly, squeezing out dozens of new viruses each second. Some of these viruses escape the cell and infect other cells, causing disease in the process. Infections from viruses include the flu, herpes, Ebola and Zika, as well as the common cold.
Scientists are continuing to study viruses and how they behave in order to prevent and treat disease. Right now, scientists are trying to understand the novel coronavirus behind the current global pandemic.
It is believed that viruses evolved from simple molecules of nucleic acid that escaped from the genes of larger entities (bacteria, plants or animals) billions of years ago. They are also thought to evolve rapidly in the body of their hosts, a process known as mutagenesis. The result is that the virus changes over time and mutates to be more or less infectious. This is how new strains of virus appear, for example, the recent outbreak of COVID-19 from the B.1.1.7 variant of the virus originally isolated from bats.