A large part of system infection with malware/spyware and viruses is because individuals are accessing the internet without proper protection on their computer. If a system is properly protected by a good internet security software and kept updated then most infections can be avoided.
Just like when in public the need for being vigilant of your surroundings while online you need to be vigilant as well. Be careful on what you click on because it could be malicious software gaining access to your system.