Imagine that you open your letter box, and inside there’s a crumpled-up paper ball and a letter addressed to you.

You find the paper ball suspicious, so you dump it, and you take the letter in. You open the letter, and its from your aunt, who is one of those chain letter types, and she’s sending this nice, handwritten postcard asking you to make 2 more such postcards and send it to people you love and care about.

Since it’s from your aunt, and you figure why not, after all the effort she apparently went through to put it in an envelope and all, and now you’ve made 2 more such postcards and sent them along to your friends/relatives/neighbors.

The letter in this case is like a virus because it has the instructions needed to make more of itself (the genetic material) and a packaging that it comes in (the envelope) that allows it to enter your house (your cells).

It’s not alive in the sense that it can’t make more of itself, by itself. The letter can’t make more letters without you (a cell), your ability to read the instructions (the enzymes that can replicate the genetic material and read it), and the printer/ink/paper in order to actually make the new letters (your ribosomes and DNA or RNA polymerases, depending on the kind of genetic material in the virus).

Now realistically, if the letter were to function like an actual virus, it might have instructions instead to "Print more copies until your printer catches on fire and explodes" and "Mail the letters to every person you personally know", and after this letter gets around for maybe a day or two, the law enforcement shows up to bust your chain letter syndicate by burning your house and all affected houses down (which is kinda analogous to the programmed cell death process of apoptosis), assuming your house wasn't already destroyed by that aforementioned printer. But that's a discussion for another day.