Hi Yoyo,
Since you seem to have missed out on some vital education and your Oxford-educated friend (or not? you don't follow each other on instagram, so I do wonder why she came to the party) Grace has failed to teach you (shame about that, you'd think they'd cover it in her multiple grand a year education at school), here is some special news for you!
A virus is a tiny little organism that does very bad things. They can be spherical (that means round, but 3d) - a bit like one particular virus whose name you might be familiar with - the coronavirus! A virus can get into animal or plant cells and start to make more copies of itself, which is reproducing - like all life on Earth (that's the big planet we live on - it's like a big sphere [no, it's not flat]) tends to try to do. When a virus does this in an animal or human body, it makes the animal/human/plant very sick. If they do it too much, you can die, which is very bad.
Viruses like to cosy up in spit and body fluid and sometimes can even float through the air. If you touch something with a virus on it and put it in your mouth, it comes into your body and starts to make copies. That's why you see those signs in toilets asking you to wash your hands after you go for a wee/poop. If you already have other viruses, or other things that make you poorly, viruses making copies can be even more dangerous, because your poor body can't fight them all! The more people in one place, the more likely it is that there will be viruses around, and the more likely it is one of them will get the virus in their body.
I hope this made it easier to understand why the coronavirus is bad and why it's bad for lots of people to be in one place when the coronavirus is around!