It's pretty obvious why Fauci took the Fifth. If he answered the questions and dissembled, lied, or couldn't keep his lies straight and without solid proof, he was looking, potentially, at getting felony charges. His best course of action when trapped in a situation where he is likely guilty of felonies is to not answer anything. Lois Learner (remember her) did the same thing when caught slow playing and denying Conservative groups tax exempt status by the IRS.
Fauci is hoping to run out the clock, and given his age he likely will. That doesn't change that it's pretty damn obvious he's:
A. Marginally competent in his field
and
B. A total piece of human shit.