"Infested" is not a passive voice here. It's a participle used as an adjective.
Why can't we go somewhere that is not infested with grizzlies? [the sentence is already in active voice, not passive] >
Why can't we go somewhere that is not full of grizzlies? [not passive]
Some other adjectives have also developed from past participles, such as "afraid."
"impressed with fear, fearful," early 14th century, originally the past participle of the now-obsolete Middle English verb afray "frighten," from Anglo-French afrayer, Old French affrai, effrei, esfrei "disturbance, fright," from esfreer (v.) "to worry, concern, trouble, disturb," from Vulgar Latin *exfridare, a hybrid word meaning literally "to take out of peace."
Online Etymology Dictionary
· 4 weeks ago