For this question as the first and second blanks are clear for me i used block of 4 strategy , i directly had put malignant and insufficient corresponding to shorter time in the second statement(the one after yet)
But i still do not understand how the third blank is “benign”
I thought Will’s statement contrasts second statement due to YET. So will says there is a relation between harmfulness and shorter time, so i also choose the third blank as “Virulent”
Why is that wrong?
When you look after the Yet in the 2nd sentence it is saying that most harmful P species has been in humans for a shorter time . So short directly relates to harmfulness in Will’s vision.
Now look at the 2nd and the 3rd black its start with therefore have insufficient time to evolve towards harmlessness --> as they have recently entered humans and we know short = harmful
Hey sorry, i still did not get it. the sentence has pointed out that short time corresponds to harmfulness , then when time is insufficient , it is harmful but how is it harmless… 
This one requires some mental gymnastics
The overall passage is saying “Wills argues that the longer a parasite species has been exposed to/lived within its human host, the less harmful the parasite is to the host.”
(Presumably because humans adapt to it, or learn how to manage it without it blowing up their immune system.)
What might make a recently-introduced parasite so dangerous is the fact that humans and recent parasites haven’t had time adapt to (evolve with) each other and coexist peacefully.
That’s why you want benign.
ahww , yeah i think i get it now, so evolve(adapt) is the keyword there ! Thanks a lot , god this question is difficult 
