You have a wrong OA. What is the source of these questions.
Check this outhttp://www.gmatclub.com/phpbb/viewtopic ... ear+rabies
I think the answer is B.
Although the fear of rabies is an on-going phenomenon, thus a simple present tense should be used, I think only B makes sense.
Few people are KNOWN to have recovered from the disease
Few people are KNOWN to recover from the disease
I think the usage of KNOWN implies that the fear of that disease was based on incidents that have been reported in the past, so a present perfect tense should be used.
If the author were simply stating a fact that only very few survive, then he would have written "few recover from the disease" rather than "few are known to recover".
Just my 2 cents.