Background Search Engine Optimisation (SEO) is a term used to describe a variety of techniques that are used to boost the ranking of web pages in the results listing of search engines. These techniques are being abused by spammers and scammers to increase their own SEO web pages so that they appear high up in any search result. The resultant web page is used to display malicious content and automatically download and run malware on the user's computer.
Note: this will affect all downloads not just trojans!
How to prevent Safari from automatically opening files