In a test an unprotected PC with no antivirus program or firewall was infected in eight seconds. Within five minutes CPU usage was 100% due to downloaded spyware, and in 30 minutes it had crashed.
This is on a Windows-based PC. This would not happen with Linux or a Mac which don’t even need firewalls.
If you have a router, many of these attacks would be stopped, so the test is a bit theoretical, but still, it’s getting ugly out there.