I have only one reason to say that B is wrong
=> According to MGMAT, Present perfect tense is for action that started in the past but continue into the present or the action that might be over but its effect is relevant to the present.
Here in option B, "have appeared" should be just "appeared" because "have recovered" is happening now. We cannot have "have recovered" while "have appeared" action is still continuing.
So I recommend action A
2 reasons I feel A is wrong
1. Fear is well founded. Its based upon the past.
2. (A) few people are known to recover from the disease ---> You must exclude the people who have the symptoms now and still recovering – to make such a statement.
people are sampled from the past
not the present
. "have recovered from the disease" looks alright.
Can you please clarify? I think it is A, too. But I am not very sure why B is incorrect.
In option A, "..to recover" seems correct because it is a fact and is still true. The usage of "...to have recovered" in option B seems to refer to events in the past.
Also in B, "once" is used rather than "after" (I read somewhere that "after" is preferable). Additionally, "have appeared" should be simple past "appeared".