In the sense of using up memory, divulging crucial information, and generally wreaking havoc on your PC, they are normally harmless.
The functional word there is normally.
Most of the info contained in a cookie is harmless - but there are exceptions, especially in ad-heavy unregulated environments (like naughty sites, "deep web" or "dark web" applications, and other things you generally wouldn't want your parents, spouse, or kids know you're looking at). These types of cookies, while not technically viruses, can be used to send dozens or even hundreds of ads to your PC based on sites you've visited, and THOSE sites can contain harmful malware that can infect your system.
It's usually considered good practice to "clean out the cookie jar" - to delete all cookies, wipe your browsing history, and remove any active logins - to prevent a gradual buildup of digital info "sludge" that can slow browsing speed and make your system vulnerable to intrusion.