Conversation so far:
When did Jonas Salk start doing polio research?
1948
What was the first part of Jonas Salk's polio research?
Harry Weaver, director of research at the National Foundation for Infantile Paralysis, asked Jonas Salk to find out if there were more types of polio than the three then known.
Did Jonas Salk find out if there were more than three types of polio?

Question: What did he discover in his research?
Self-contained rewrite: What did Jonas Salk discover in his polio research?