While most scientific findings are the result of deliberate and carefully designed experiments, occasionally a major discovery happens in a more fortuitous way. One such example is the discovery of penicillin in 1928 by the Scottish bacteriologist Alexander Fleming. After working as a field doctor during World War I and witnessing how many soldiers died needlessly from bacterial infections, Fleming recognized the exigency of finding a chemical that could fight these afflictions. He began to conduct experiments by growing bacteria in Petri dishes and treating them with various chemicals. Unfortunately, none of the chemicals he tested proved effective at stopping the bacteria's growth.
What is the meaning of exigency as used in the passage?