It depends on how fit/active they are but there is no set amount I would say at least 10
For example, people who have anorexia severely reduce their food intake to lose weight. People who have bulimia eat an excessive amount of food in a short period of time, then purge or use other methods to prevent weight gain.
Our immune systems have improved over the years due to improvements in the medical fields. I also believe your brain has a bigger effect on how sick you actually allow yourself to become
