There are many different types of viruses. Some need a host programm to work properly (this can be any type of programm). Some are known as worms and can run as is and spread itself further through your network. And then there are the trojan horses. The trojan horse is a story of how the Greeks created a wooden horse to win the trojan war. They hid soldiers inside the horse and then "ran" away leaving the horse and Sinon who persuaded the Trojans into believing that it was an offering for a G.o.d. Although the horse seemed suspicious, it was brought into the base. At night the Greeks emerged and opened the gates for the returning Greeks. These kinds of viruses claim to do one thing whilst doing another. The worst kind are the fake anitmalware viruses that trick innocent, unknowing people.Virus hoaxes are also dangerous, although they are technically just hoaxes/fake. These types of viruses are harmless but can cause people to ignore actual viruses.
A while ago there was a steam game that was a pretty simple and neat platformer. The problem was however, that the game made GPUs (graphics processing unit) and CPUs (central processing unit) struggle to keep up. After a while it was discovered that the game actually hijacked computers to farm cryptocurrency.
Although it is recommended to load a backup if your computer gets infected, personally I believe the only way to truly delete/remove every virus is to buy a new hard drive/ssd. However most of the time a reformat will do wonders and at least speed up your computer. If you plan on transferring old files, try to only bring what is absolutely necessary as viruses can be hidden amongst normal programs. Also most importantly, don"t click on unknown links or links from unknown sources (links can be edited to show another website). Never download anything from untrustworthy sources. Be careful of random USB sticks that you might find or ones that you receive. Ignore any email that appears to be a scam.