A: 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."

Hackers exploit security vulnerabilities of web software (blogs, forums, CMS, image galleries, wikis and e-commerce solutions) to insert hidden illicit content into web pages of innocent third-party web sites.

Most times this technique is used by spammers who insert hundreds of hidden links to web sites advertising medications, cheap loans and porn.

Another example of hidden malicious content is code that redirects visitors that click on your site's search results in Google to absolutely different web sites owned by spammers. This sort of exploit can hardly be detected by site owners, because the malicious code redirects only first time visitors.

What all these techniques have in common is they parasitize benign web sites to take advantage of their search engine ranking, visitors, and site hosting which is paid for by their victims - unaware web site owners.

Read my Introduction to Website Parasites

A: The fact that you don't see them doesn't make them less harmful.

• Parasites steal your site visitors redirecting them to other site
• They make your site less responsive.
• They steal your bandwidth.
• Hidden links are rightfully considered as spam by Google. It's just a matter of time when your site will be removed from Google's index and your AdSense account will be blocked.
• The mere fact that hackers managed to incorporate their code into your web pages mean they can use your site for all sorts of illegal activities: from sending spam e-mails to attacking other web sites. And you will be liable for the damages incurred by such malicious activities or will have to prove that your site was hijacked by hackers and you are not one of them.
• Any private information stored on your web server (such as passwords, client data, etc) is now exposed to hackers.
• Even if the parasites are left unnoticed and search engines don't ban your site, every time you pay your hosting, you are sponsoring spammers since you are sharing your site with them. Just think about it.

A: Parasites do their best to remain unnoticed. However there are various symptoms that can tell you you should check your web pages.

• Your web site became less responsive. It takes longer to load web pages.
• You see a sudden decrease of visits from search engines.
• People find your site in search engines using absolutely irrelevant search terms.
• Your site or some particular web pages have been removed from search engines.
• Your AdSense account is blocked.

A: I hope you are right. But unless you check HTML code of your web pages you cannot be sure. Use the Unmask Parasites service to reveal suspicious content hidden inside your pages.

A: Major risk factors are old versions of popular web scripts used on your web site. I.e. CMS, blogs, forums, image galleries, e-commerce scripts. Always upgrade to the latest version when it is available.

Another threat is site templates and themes from unreliable sources. They may contain pre-built hidden links and even trojan scripts.

A: No. Hackers are interested in all sites. No site is too small or too large for them. Of course, if no one knows your site and there is no links pointing to your site, you are pretty much safe.