Ixodidae's constant ability causes it to be trashed before any conditional abilities can trigger off of the virus counter purge. The Runner does not gain credits from Ixodidae's ability if s/he has Fester installed.:Ruling
Does Ixodidae trash before it gives you money from Fester when the Corporation purges virus counters? Ixodidae and Lamprey have constant abilities ("Trash if the Corp purges…"). Even though these require a certain condition to be met, “if” means that it is always active and resolves before triggered abilities. So the Corp purges, the Ixodidae trashes, and then the Runner resolves the Fester. Since the Ixodidae is no longer around, the Runner cannot gain credits from the conditional.