Unsolicited software that is installed in a computer without users realizing it. There are many different types. Parasites can report Web browsing habits to a marketing company over the Internet (see
spyware) or change browser settings to point to a specific site. They can redirect search engine results to a site that sells a related product, and they can cause premium services to be dialed up.
Read the License Agreement
Parasites are often installed with freeware, and the license agreement may actually say so, but hardly anyone reads it. Sometimes, users can opt out of installing the parasite and install only what they wanted in the first place. Be sure security settings are set to medium at least, and never click "Yes" to any dialog that asks "do you want to run" or "execute" something unless you know what that something is. ActiveX controls on the Web cannot only install parasites but viruses as well.
Parasites often do not include an uninstall function and may not be easily removed, although anti-parasite programs can detect and remove them (see
spyware blocker).