There has been considerable online press coverage recently about server vulnerabilities, websites hosting malicious code eg virus in HTML, and hacked websites generally. (Actually, the correct term is ‘cracked’ as ‘hacked’ just means anything to do with coding, rather than nasty code).
If you run a commercial website, it is vital that not only is it working as close to 100% of the time as possible, but that it is not serving up anything which could put your potential customers off eg content or code posted by someone who has unlawfully accessed your servers.
Many companies, understandably, use hosting companies to provide all these services, but it is worth regularly checking how up to date your hosting provider is with security flaws, patches, vulnerabilities etc.
Also, you should use Google Webmaster tools to check that your site has not been compromised in any way. Google have this week updated their webmaster blog about some of the most recent issues they have encountered, and offer some solutions to the problems.