The Google Security Blog reported that malware is a growing issue. A Google study showed that a single malware source has infected over 60,000 hosts. The chart below shows the number of infected sites over time from the top ten malware sites.
Why is this a SEO issue? Well, it might not hurt your rankings but it will 100% hurt your traffic from Google and traffic in general. Google labels malware sites with a big warning on the Google search results. It looks like this:
Trust me, you don't want malware on your site and you don't want that malware warning.
For our past articles on malware, see here and you can learn how to remove the malware and remove the Google label.
Forum discussion at Google Webmaster Help.