Generate a question which can yield the answer mentioned in the input. Generated question must be answered by the answer provided in input, without using any extra knowledge.

Context : Ludwik Gross (September 11, 1904 -- July 19, 1999) was a Polish-American virologist who discovered two different tumor viruses, murine leukemia virus and mouse polyomavirus, capable of causing cancers in laboratory mice.
Answer : virologist
What was Ludwik Gross's occupation?