(19:22) 'Ki lo uchal' - Rashi says that the angel in charge of destroying Sedom was forced to say that he wasn't able to do it, because he had previously told Lot that it was in their hands (ki mashchisim anachnu). Therefore, Hashem forced the Malach to admit that it wasn't actually dependant on them, and it was Hashem who was in charge.
If we get too cocky or complacent and tell ourselves that we are in control and we can defeat this disease (mashchisim anachnu), Hashem may arrange circumstances so that we are forced to admit it is not in our control and we are powerless. In the words of the angel - "ki lo uchal" - I can't do it myself, its all Hashem.