Simplify the following text for a Grade 11 native English-speaking person.
Original: It was not until the proofs of Robert Koch ( 1843-1910 ) published in 1891 , for which he was awarded a Nobel Prize in 1905 , that microorganisms were confirmed as the cause of infectious disease .
Simplified: It was not until Robert Koch 's 1891 proofs , for which he was awarded a Nobel Prize in 1905 , that microorganisms were confirmed as the cause of infectious disease .