In no modern Operating System viruses are an issue for the sensible user. So viruses are an issue if you're not sensible.
Viruses in fact don't get harmful unless you actually run them. That's almost the same in every different O.S., and do mistrust people telling you "Linux is good because Windows has got viruses" because it's nonsense. Linux (and Ubuntu) is good for many reasons, but this has nothing to do with viruses.
However, running Ubuntu or Linux in general lets you be statistically less prone to get infected by viruses. Ubuntu encourages users to choose official repositories as the main source of software (which are safe), and running an executable by double-click is less easy than in Windows.
But you can still get into a website that asks you to add a poisoning repository, or to install a malicious .deb (with double-click) to add some awesome functionality Ubuntu is missing.
So viruses are a problem because of the user, not because of the system (which in most cases is an innocent victim), even if in some old Windows systems (95, 98, ME but also old versions of XP) one could get a virus without doing anything unsafe, just through some vulnerability of the system. But leaving out pointless chat topics (Micro$oft is baaad, etc...) nowadays Windows systems are safe enough for the use cases they're intended and do get infected because of user inexperience and IT unknowing.
So, to answer your question:
- Windows users are usually less competent users and get infected with ease
- Ubuntu users use repositories which are safe
In no way Ubuntu is stronger towards viruses, I can write one tomorrow, pack it in a .deb, and it would be insidious and destructive also because people think "linux has no viruses". Linux has no viruses until you create one, which can be done in minutes, assuming you can convince the victim that it is safe to execute it.