The invention and use of the microscope showed scientists that germs could cause illness. although there had been various understandings before for why people become ill, the discovery that microscopic germs can make people sick completely changed scientific understanding of why diseases exist and spread.

this change is an example of what?

how a law changes when new evidence disproves it
how a law stays the same as new scientific inquiries are made
how a theory changes as new areas of science open up
how a theory stays the same when new research confirms it