Viruses are not alive, at least in the classical sense. While they’re made of proteins and genes like living things, they need to interact with living host cells to reproduce. These agents of cellular mayhem have been the cause of history-altering outbreaks and pandemics, from smallpox and polio to HIV and Ebola, but were only discovered at the turn of the 20th century. Since then, we’ve found them in nearly every ecosystem worldwide. Viruses are, and always will be, the world’s experts at going viral.
So ... What Is a Virus?
Viruses are ultra-tiny packages of genetic material. A single particle, or virion, of influenza is up to 100 times smaller than common bacteria; you could fit some 15,000 end-to-end across the head of a pin. The outer layer is a protective shell called a capsid; some viruses also have a viral envelope, a second layer that helps virions attach to host cells. The envelope can act as a sneaky cloaking device, helping virions avoid detection by a host’s immune system. Most viruses have just a few genes, which contain the instructions for making new viruses in either DNA or its single-stranded relative, RNA. But they don’t have any cellular machinery to read and execute that genetic code. That’s where a living cell comes in. When a virus bumps into a potential host, proteins on its outer layer interact with proteins on the living cell’s outer membrane. If it’s the right type of cell — for example, most influenza viruses can only infect certain cells in your nose, throat and lungs — it’s able to latch on and inject the cell with its genetic material. The host cell doesn’t realize the new genes are foreign, so it runs the instructions written in the genetic code alongside its own. This tells the cell to make copies of the viral genome and package them up into new viruses that burst out of the cell to find host cells of their own. Many viruses replicate through this process. However, when an RNA-containing retrovirus infects a host cell, the RNA is converted to DNA and then inserted into the cell’s genome. This extra step in the replication process creates more room for a copying error, which makes the retrovirus more prone to mutation and rapid evolution. Researchers have yet to develop a vaccine for the retrovirus HIV, for example, in large part because its multiple strains keep evolving.
