"Wikipedia defines Parasitism as a “type of symbiotic relationship between organisms of different species in which one, the parasite, benefits from a prolonged, close association with the other, the host, which is harmed.” This definition perfectly describes relationships between hackers and legitimate websites. As it often happens in real life, the host (legitimate website and its owner) may be completely unaware of parasites until the harmful effect becomes obvious (e.g. drops in traffic, lost search engine rankings, site gets blacklisted, etc. ). And it doesn’t matter how big or small your site is and how malicious the hack is – this is the sort of relationships where parasites (hackers) always win and legitimate websites always lose.
As a webmaster, you can be more effective at detecting and mitigating parasitic activities if you know how hackers can benefit from your site ." – Content courtesy of Introduction to Website Parasites | Unmask Parasites. Blog.