A parasitic affiliate earns commissions he doesn’t deserve because he doesn’t have really influenced or provoked the purchase decision. To see more on the topic, see affiliate parasitism.
Main types of parasitic affiliates are:
Softwares used by parasitic affiliate and installed on desktop users are also called parasitwares by the affiliate community.
If parasitic affiliates are logically decried by "traditional" affiliates, they are most often accepted by merchant programs. Sometimes, for a given parasitic affiliate, some merchants pay commissions for sales through the website but not for sales driven from the toolbar.
Parasitwares are an issue, because, most often, they inject or overwrite affiliate links and cookies when the user is on a merchant website with an affiliate program.
An example of alert with a loyalty toolbar:
A video example of an affiliate parasite: