Why is heating and cooling necessary when doing the pcr method?

A PCR is used to amplify DNA. The heating breaks the hydrogen bonds and ensures that the double-stranded DNA helix is melted such that the two strand separate. The cooling ensures that the introduced DNA primers bind with the single strands so that elongation of the chain (to form a new double helix) can be possible by DNA polymerase.

it would lead to upper respiratory infections and pneumonia.

that scientist would be robert hooke