OS X El Capitan: malware
Malware is malicious software that includes viruses, worms, trojan horses, and other programs that can harm your Mac or your privacy. Malware can unintentionally be installed when you open attachments in email messages and download items from websites.
Some malware is simply annoying. More often, its intent is to control your Mac to collect personal and financial information, host illegal content, send spam, or harm other computers on the network.
Avoid opening or downloading items from email messages and websites unless you are certain that they come from a legitimate, trusted source. If you’re uncertain about the source, delete the item. You can always get it again later, after you make sure it isn’t malware.