Once executed, a virus finds another program and attaches itself again. As with any life form, its imperative is reproduction. But life forms don't only reproduce. Every life form entertains itself somehow. If a cold virus merely attached itself to you without making itself known, you probably wouldn't mind it very much.
Viruses refuse to keep quiet.
They might grab some of your memory. Not much - perhaps a few thousand bytes taken directly from the operating system. The operating system controls all the memory for all other software use, including its own use. As a passive lender the operating system expects that well behaved programs will borrow some memory for a while and give the memory back later. The operating system doesn't knock on the door and ask for the loan's repayment.