It's now "one of the world's most urgent health crises", say WaterAid and Tearfund.
The children who suffer most live in overcrowded cities, like Bangladesh.
Millions of people move from the countryside to cities every year. Because there's no room for them, they have to live in slum areas with no toilets or clean water.
They have to go to the toilet in the streets. So any water they have to drink gets polluted.