He has no way of demanding payment up front. He can maybe trust them to realize that someone who knows one thing might know two and someone who doesn't get paid for one favor doesn't do two. They're not essi.
"Caralendar pox, distemper, rabies, and many other diseases are alive. Cholera, blue plague?" He shrugs. Trying to suggest maybe. Probably. "Alive cholera." He cups his hands and mimes drinking from them. Then he mimes vomiting. "Not alive cholera." He mimes drinking and then doesn't mime vomiting. "Mahan... not alive." He makes a wild guess about which part of "other diseases" means what. "Many other alive not diseases. Many other alive..." Mahan smiles and makes a point of looking healthy. "Many other alive later not alive... person not alive. Many, many, many other alive not diseases. Many alive, person, Spite..." Mahan holds his hands out half a foot away from each other. "Diseases and many, many, many other alive..." Mahan brings his hands all the way together, hunches over, tries to suggest very tiny as hard as he can. "Diseases not alive, many other alive..." Mahan takes a breath, tries to think how to say it, gives up, switches to Hari. "Look, I can't explain sanitation in a language I don't speak. Please figure out that this is important enough to pay a translator."