Consider that student had had a strep throat two years ago, why would she develop it again?

Yes,a strep throat can reoccur in that student because of the factors like -

Explanation:- Strep throat is the situation of the throat when it feels pains and inflammated. There are chances of the relapse of strep throat because of the weak immune system and does not complete course of antibiotics or has resistance towards it .The student can also catch the infection again and can still have presence of the bacteria of strep throat .

your answer is "the 1950's"

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.