‘Parasite’ is a shorthand term for "unsolicited commercial software" - that is, a program that gets installed
on your computer which you never asked for, and which does something you probably don’t want it to, for someone
Many web users unknowingly have unsolicited commercial software installed on their computers. This can cause
system instability, add unwanted advertising, spy on everything one does on the web, or compromise security.
The parasite problem has grown enormously recently, and many millions of computers are affected. Unsolicited
commercial software can typically:
plague you with unwanted advertising (‘adware’);
watch everything you do on-line and send information back to marketing companies (‘spyware’);
add advertising links to web pages, for which the author does not get paid, and redirect the payments from affiliate-fee schemes to the makers of the software (such software is sometimes called ‘scumware’);
set browser home page and search settings to point to the makers’ sites (generally loaded with advertising), and prevent you changing it back (‘homepage hijackers’);
make your modem (analogue or ISDN) call premium-rate phone numbers (‘diallers’);
leave security holes allowing the makers of the software — or, in particularly bad cases, anyone at all — to download and run software on your machine;
degrade system performance and cause errors thanks to being badly-written;
provide no uninstall feature, and put its code in unexpected and hidden places to make it difficult to remove.