Question about Delhi search (spoilers)
Play the text on the card to the letter. If the card says infested, it means infested. 

Thematically the Vector Control Technician helps control the vectors that transmit diseases, such as rodents, birds, other animals, insects etc. They're like a specific pest exterminator, that only target the species that carry the disease. If you keep that in mind, it makes sense that they're so useful when the disease is being carried by humans. 

If you can't find official errata that supports a widening of scope for a card, just play the card as is.