Have you ever gotten any emails saying that your website has malware on it or your anti-virus program has triggered a security threat? Google WebmasterTools emailed you about this as well? The issue could just be a bad link, bad coding in a plugin/template or a poorly coded theme that you are using. There is so many variables that I could mention to why malware is on your website but what you could do is, have it monitored 24 hours a day, 7 days a week if you are running the