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You build up your immunity when exposed to an unclean environment but it will wear off quickly.
People who live there have much higher levels of immunity and can be carriers of many pathogens and still look healthy. They will make you sick even when they don't seem sick.
The place is probably loaded with pathogens including the air that you breathe.

I guess you're right. It has been more than a year from the previous time when I went there. My immunity to the place may have reset.