We had the iMac for 7 years and not once did I have a problem with viruses and there was never a hint of system degradation.
In contrast, my wife had to replace here PC laptop 3 times due to a nasty virus we got from using it in Spain while we did not have an anti-virus program. The other times was due to the system just slowing down.
It all depends on how you look at it. It's true that a Windows OS is more vulnerable than mac. There are two reasons windows is infected more often than Mac.
Firstly, It's because Mac has all the doors closed. The user is much more limited. Windows keeps a lot of doors open to give users more power and flexibility on their OS. Registry Editor, Group Policy, Management Console and a lot of more tools with thousands of options to be able to customize your OS as you wish. There is also a
God Mode in windows. Try it once on your wife PC and compare it to your Mac. When an OS gives you more options to play with and customize it's more vulnerable comparing to a more closed OS. You can go with a more limited OS if you need to be safer, Or go with a more open one if you need more power and freedom on settings and customization (Specially when you are in a network or domain)
For example, this is the settings for a PC password in GPedit.msc
Which one of these options you can customize in your Mac?
Users can open their PC case and replace their graphics card or processor with a more powerful unit. With the right parts, a relative novice could follow a guide and build his own PC. In contrast, it's nearly impossible to build a computer capable of running Apple's OS X operating system and software. In their rare appearances, such machines (often referred to as "Hackintosh" computers) require a precise mixture of the correct components and a hacked version of the operating system
There's one more reason that makes Windows more vulnerable.
First look at this Worldwide Desktop OS Market share image from
https://gs.statcounter.com/os-market-share/desktop/worldwide. (or you can google and check other sites)
You can see the difference. So if I am a hacker and want to write a virus I would target the virus to attack a windows not a Mac. Because it holds almost 80% of world wide market share and the possibility to find a hole to infect it, is much more than 10% of Mac. How many hackers will try to write a virus to infect an OS with 10% market share?
I'm using windows OS since windows3.11 in 1992 (I'm not sure about the date). It's more than 28 years. I've never used a virus scan software and have never been infected. If you know your windows and how to manage things, you'll be safe.