Basically, the chances of getting illnesses from food are related to your usual eating habits. The body develops its own immune system over a period of time.
If your maasi cooked unhygenic food, and your body had developed anti-bodies to fight these bacteria than rehree food probably won't do you any harm either. Same is true for tramps of New York or Lahore.
As a contrast, if, e.g. an American born-and-bred boy comes to Pakistan, he may have severe illness just by drinking tap water in Karachi. His body just doesn't have the anti-bodies to protect him.
Most middle-class and upper middle class families in Pakistan have reasonably decent hygenic conditions maintained in the home kitchens. So kids are protected from these bacteria and virii which are often part of stale and uncovered food or carried around by flies, dust and other insects. Also the raw materials used by rehree walas are also highly dubious and certainly not bought from top-of-the-line grocery stores.
These families warn their kids to stay away from rehree food to avoid getting ill. On the flip-side, if you ignore this advice, and eat a lot of rehree food, then initially you may get sick, but over a period of time, you may develop an immunity in the body.
Most school kids, develop a sort of immunity by eating kulfis, imli, gola gandas, cholay, shakar-qandi and chalee (bhuttas, for some) from the rehris in front of their schools. The chalee itself is quite paak saaf, but the masala applied to it has several questionable raw materials. Same with shakar-qandi.
Be warned, though, that constantly testing your body's immune system can cause other severe medical problems. A friend of mine, who practically lived off rehree food and Lakshmi chowk kay cholay, had kidney trouble 7 years ago. Then both his kidneys failed. He went on dialysis. Got a donated kidney, but his body rejected. Six months ago his brother donated another kidney, and now he is better. He is a gold-medalist in MBA, but during this ordeal he lost his study career, lost three jobs, one after the other, and is now just a skeleton. His last three operations were made possible by donations from all his friends and former employers.
[This message has been edited by Pristine (edited April 03, 2002).]