Yeah 900 units of food would typically last a family a lifetime, and you did cure their illnesses which means it just couldn't have been helped this time.

Sometimes even after they are cured the disease will come back because they are suffering from a serious or grave infection which is when a doctor consultation would be helpful.
Now that I have over $500,000 in this family I give anyone that gets sick the doctor consult.
Today the 6 year old in my game was sneezing and the rest of his family was sneezing as well, but he was the only one with a Weak status. I used the consult on him and it said he has a serious infection and needs the penicillin.