Derzooger, icot can probably answer this better but I’ll try 🙂
If you have anti-virus software constantly watching your computer it is far less likely for you to be infected by simply openeing an email or web site. In order for a virus to infect you simply by opening an email, your email client has to run the code. I think that Gmail generally guards against this by not running any code in an email, they even block images which can sometimes contain malicious code (besides the obvious issues they may contain for a ben Torah). I would think that an email client like Outlook that is set to download the entire email is more likely to have such issues which is why you should have some sort of security software that scans emails as they come in (Gmail does this automatically for attachments though it’s not foolproof so don’t open suspicious attachments even if Gmail or any other program scanned it)
In terms of web sites, you often have to click some sort of confirmation for it to affect you. These confirmations are often hidden in innocent looking yes/no type popups or more tricky ways such as the one described in the above article.