Why does Grandfather think he has been a fool?

Grandfather thinks he has been a fool because he failed to see the inherent threat in the yellow fever. In Chapter 5, Lucille (Matilda's mother) wanted to send Matilda to the Ludingtons at Gwynedd. However, Matilda's grandfather would not hear of it; he maintained that the Ludingtons were not family and that Matilda would fare better by staying put.
Matilda's grandfather believed that they would only need to accommodate the fever for a few weeks. He insisted that life would soon return to normal and cautioned against panicking.
In Chapter 11, Matilda and her grandfather are abandoned after the armed men determine that the latter is too sick to be allowed passage through their town. The two are left without food and water, and Matilda's grandfather eventually laments his foolish stubbornness. He admits that Lucille was right to worry and that they should have left town before any of them caught the fever. So, Grandfather thinks himself a fool because he resisted the idea of moving.