A pandemic more often – how global warming may awaken long gone viruses:

What are viruses?
Viruses are microscopic organisms that require hosts such as plants, bacteria and animals. A virus is typically 20 nanometres to 400 nanometres in diameter. Like all other organisms, a virus is made up of DNA (Deoxyribonucleic acid) or RNA (Ribonucleic acid) but is surrounded by a defensive coat called a ‘capsid’. Some viruses may also be covered by a second coat called an ‘envelope’ that produces spikes. According to the ISRRT, when a virus invades a host, the organism gains access to cells and uses the machinery within, as a factory to begin manufacturing more of itself.