Email, to begin with, and at its core, is basically a text-only messaging service; as such, it did not previously pose a serious security risk in terms of: viruses, trojan horses, malware and others nasties. However, the email format was expanded - via MIME - to include additional features such as imbedded HTML and attachment files.
This expansion of the email format - whilst intended to offer users a more expansive service - had the knock on effect of posing a far more serious security risk. Imbedded activeX components in the body of email messages, hyperlinks imbedded in the body of email messages, and, of course, file attachments, could pose as harmless, but, infact, be a virus etc.
It would be fair to say that in the 1990s the majority of virus infections were the result of infected attachments in emails. Thus, email providers had a huge headache, and the solution was to imbed an anti-virus scanner into their service. Present day, every webmail service worth its salt - likewise for anti-virus applications - will scan every email for viruses. It's not uncommon for email providers to employ experts like McAfee and Symantec to provide their anti-virus protection. Is this to say it's impossible to receive a virus via email? no, but there is certainly more protection than in the 1990s, when it was common place.