Why so many sites are infected?
WordPress has evolved from a pure blogging platform to a popular and full-blown content management system (CMS), and many people are making business out of it. By using the power of WordPress plus paid plugins, such as Slider Revolution, and premium themes, people without programming skills can design web sites for companies. Apparently, Slider Revolution is a popular paid plugin and many web site developers have purchased this plugin to add photo slide effect. What is worse is that this plugin has also been bundled in many premium themes. Although the directly purchased plugin can be automatically updated to patch any security vulnerability, the one bundled in themes must be updated by the theme developers. But because the developers of Slider Revolution decided not to expose the security vulnerability to the public, those theme developers were not aware of the risk and didn’t update the bundled themes.
How do I know if my site is infected?
Sucuri Security has released a free scanner that can scan your web site to let you know if you web site is infected or not. Check your web site with Sucuri SiteCheck.
What if my site is infected?
Honestly, it is very hard to clean the malware, because it is injected on every page of your site. Even you manually cleaned the code, you still need to deal with the backdoors that hackers have installed on your site. So I would honestly suggest you to do a full restore of your site from your backup. You do regularly back up your site, do you?