Can your iPhone give your PC viruses when connected?

Is it possible to give your PC a virus from your iPhone?

As I may have accidentally downloaded a dodgy app and I did a scan recently on my computer with Avast and something came up called "Rogue.Multi"

I read that this can be caused from downloading dodgy programs intended to do harm… But I don't recall downloading anything I wasn't sure about on my PC. So i'm wondering where this virus came from…

Well, basically yes.
If the malware is Windows-based, once you connect your smartphone, it acts like a thumb drive, and the Autorun makes the malware to sneak into your PC very easily.

The first thing you need to do is to Turn off Autorun and Autoplay if they're turned on your Windows OS.
Then keep your antivirus, antimalware UP-TO-DATE and make sure you ENABLE the "On-Access-Scan" and "On-Demand-Scan" on your antivirus/antimalware.

Very unlikely - IOS/apple operating systems tend not to get viruses - the payoff for virus authors it much less than it is with windows operating systems