DNA and RNA are molecules that act like instructions. The virus can do this because viruses and cells have an important thing in common: they both use DNA and RNA. Once a virus enters the cell, it can use the cell to make more viruses. This starts a process that leads to the virus either entering the cell whole, or injecting its DNA or RNA into the cell. Proteins on the virus’ surface are shaped just like the keys, and fit into a receptor. Receptors are proteins on the surface of cells that act like locks. To enter the cell, a virus floats up to, or lands on a cell, then attaches to a receptor. Instead, they take over cells, and trick the cell into making new viruses. Viruses can’t make new viruses on their own. Those without one are called non-enveloped (naked) viruses. Viruses with a protective envelope are called enveloped viruses. Some viruses also have a lipid envelope that is similar to the barrier that surrounds our cells. The most basic parts of a virus are genetic material (DNA or RNA), a few proteins that will help the virus to replicate in a cell, and a protective protein shell. HIV, bird flu, SARS, and COVID-19 are examples of this. But very rarely, a virus can mutate in a way that allows it to cross from one kind of host to another. So, a virus that makes your cat sick isn’t likely to make you sick. One kind of virus usually only infects one kind of host. Plants, animals, fungi, and even bacteria can be infected by viruses. There are many kinds of viruses, and viruses infect every form of life on earth. Colorized image by José R Valverde from Pixabay. This makes them very different from enveloped viruses, like the flu. Viruses like the rhinovirus shown here (which causes the common cold) don't have a fatty envelope.

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