Kate says that Keller hasn't been sick for the past 15 years . Why does George question this statement

Mother responds proudly that her husband has not been sick for fifteen years. Keller hastily adds the exception of his flu during the war. Mother takes a moment before she realizes what he is talking about, and George notices the awkwardness. His suspicion is reawakened. Steve told George that Keller claimed to be sick in order to avoid suspicion.