Wikia does not have any viruses or malware. However, like on every other website, you should still be careful when clicking links that are taking you to other websites, and be careful when downloading anything by making sure it is from a trustworthy place. Also, if you notice an ad on Wikia that seems broken or otherwise inappropriate, you can report it here.
MichaelWilemonTheMime: Recently I encountered a bad ad that managed to automatically redirect me from the article I was viewing to a blank page on a different domain, which looked like it was deploying an exploit kit. This is not Fandom's fault but rather rogue advertisements.
If people do get malware then the ads are most likely to blame. See this blog for more info such as how to safeguard yourself, if you are concerned. Out of my few years of using Fandom this has only happened a handful of times.