Question: when you look at common viruses what is a feature that they all share? what is the most common way that they are spread?

Viruses all have certain features. The most important is that they’re what’s called ‘obligate parasites’, which means that they can’t live on their own but have to have a host. Many viruses can *survive* outside of the host, sometimes for quite a long time, but to grow and reproduce they need a host. Because of this need for a host, all viruses also have a few tools in their biochemical toolkit that they use to infect the host: a way to break into the host’s cells, a way to replicate themselves when they’re inside (which is usually a gene that makes an enzyme called a polymerase), and then a way to package up the copies that they’ve made and get them out of the cell to infect other host cells.

These are some of the common features that viruses have, though one of the interesting things about studying viruses is the sheer variety in how they implement these features. All pairs of “pants” have some common features, like two legs and a way to do them up, but we still go shopping because there’s hundreds of way to make a pair of pants that have those features! For example, when it comes to entering the cell there’s a few major mechanisms that viruses use, including membrane fusion (the virus’s ‘skin’ fuses with the cells and dumps the contents of the virus’s genome into the cell), endocytosis (in which the virus basically cuts a hole in the cell to get in), and injection (the virus finds a ‘hole’ through which to dump its DNA or RNA into the cell).

I hope that sheds a little light on the matter! There’s a lot to study here; I pulled some of this answer from a text on my desk called ‘Principles of Virology’, which is packed full of this sort of stuff – to the tune of nearly a thousand very large, heavy pages…