Here in BKK where I live, millions upon millions of people are eating street food daily... and for 2 years already, from people I know... I never heard anybody dying or getting sick. What makes street food not healthy is sugar, MSG and other additives (but the amount of it pales in comparison to your big mac).
In fact.. when I compare westerner(usually those big mac eating ones) and locals(street food ones you talking about)... it sickens me. Overweight fake food eating Americans and Europeans walking around it's easier to jump over them then it is to go pass them. Do you think that's healthy ?
What makes McDonalds and other fast food chains and food in general, not healthy is like stated above.. number of additives/chemicals in it.
Basically... you shouldn't trust anything other than 100 % natural and naturally grown product. Why I know ? Because I grew up without electricity in a war torn country. The food that I had available was 100 % organic grown by me, my neighbors and people from villages around my village. When you cut the tomato in half, your whole house smells of it. That is a tomato.. the thing you eating now if you live in any modern city, is not a tomato no matter what you may think. It's devoid of 80 % of what tomato should be. How or why I don't know. Maybe it's GMO, maybe it's been frozen 20 times .. no idea. I just know I haven't seen a tomato looong time and I miss it. As well as cheese, not that sliced shit you buy in 7/11 or wherever .. wrapped in plastic.
For the record I eat fast food like McD too and sometimes I even drink coke.