Mark Balla was on a business trip to India when he met two young men on a train. They invited him back to see their home, one of the world's biggest slums. This meeting changed the course of Mark's life. (R)

A former Lonely Planet writer and researcher, Mark was wary of participating in 'slum tourism', but something made him say yes to the invitation.

That visit changed the course of his life: it was where he learned many millions of people go about their lives with no access to a toilet - at home, work or school.

Mark became obsessed and began learning as much as he could about the situation, particularly in relation to Indian schools.

From there, he met Mr Toilet, Poop Guy, and eventually Mark earned the title of Toilet Warrior.

This episode of Conversations touches on developing world, humanitarian aid, career changes, sanitation, preventative disease, exploration, travel, schooling, education, India, Mumbai, menstruation, women's health, girls' education, infectious diseases.