Corporations are pikers compared to universities. We get hammered constantly from inside and outside the wire. You install a new PC with the cable connected and you will get infected. It’s guaranteed. If the firewall is down, you are doomed.
We have guys who do nothing but isolate infected machines and watch for bad traffic.
When I was in college last year, they decided to stop mandated installations of AVG at the beginning of the school year, and all hell broke loose. It took months to get rid of all the viruses.