It doesn’t breathe, it doesn’t eat, it doesn’t excrete, and it doesn’t grow – so it can’t be alive, can it? It hijacks a living cell and uses it to produce so many copies of itself that it bursts the cell – so it can’t be dead, can it?
What is it? A virus.
Viruses come in many shapes and sizes and infect every living thing. There are even a whole group of viruses that infect bacteria called bacteriophage. The term virus comes from the Latin for poison.
Viruses are very, very small – about one 500th the size of a single skin cell. Comparing a virus to the size of a flea is like comparing a person to the size of Mt Everest. If a skin cell was the size of an A4 page, a virus would be the size of a 10 cent piece.
Viruses are very simple, with a relatively short piece of genetic code inside a packaging of protein called a capsid. When the virus infects a cell, it inserts its genetic code into the cell and takes over the machinery of the host cell to make lots of copies of its genetic code and proteins for its capsid. The new viruses are then assembled inside the host cell and eventually burst the cell and kill it. Viral destruction of cells causes disease.
Different viruses infect different types of cells, which is why each causes its own unique disease. For example, HIV (the AIDS virus) infects immune cells, which is why patients with the HIV virus are unable to resist other infections and cancers. The cold virus infects the cells in the back of your nose and throat, which is why you get a sore throat, sneeze a lot and get a runny nose.
Viruses are grouped according to the type of genetic code they carry – either DNA or RNA – and then by the shape of their capsid. The diagrams above shows the relative size and appearance of some virus families. The family name has been given as well as one of the human diseases caused by a virus from that family.
Nature of Science
Scientists are discovering how viruses affect cells, but this changes with time as the viruses mutate. Often, scientific knowledge is tentative or based on our knowledge at the time the observations were made.