What was crisis that drove Mrs. Frisby to see Mr. Ages?

Mrs. Frisby goes to see Mr. Ages because her son Timothy is gravely ill. Mr. Ages acts as a pharmacist, prescribing medications to the ill with his advanced intelligence (which the reader later learns he acquired from his time at NIMH). Mr. Ages says he has pneumonia and must stay in bed until the weather becomes warm again, or else the chill in the air will kill him. Mrs. Frisby knows she cannot do that because the farmer whose land they live on will run the tractor through their home—every time that happens, they need to leave their cinder block house.
So, it is Timmy's precarious health that inspires Mrs. Frisby to see Mr. Ages, even though he is a bit abrupt and rude.