He could make sulfa drugs, Keltham thinks.  Probably.  He doesn't really know how, but he has a rough idea what sort of chemical that should be, they have all kinds of reaction-pathway tricks up their sleeves now.  That will make more people survive bacterial infections if they can't afford Remove Disease, and their population density will go up until they're more vulnerable to disease and things are brought back into equilibrium, unless he can first master contraception, or maybe roadmaking so that there can be more better-connected cities, which possibly need to be supported by better crops.

It's a big tangle and he knows this, so why is this a sad thought?

He could make sulfa drugs, Keltham thinks.  Probably.  He doesn't really know how, but he has a rough idea what sort of chemical that should be, they have all kinds of reaction-pathway tricks up their sleeves now.  That will make more people survive bacterial infections if they can't afford Remove Disease, and their population density will go up until they're more vulnerable to disease and things are brought back into equilibrium, unless he can first master contraception, or maybe roadmaking so that there can be more cities with lower density, which possibly need to be supported by better crops.

It's a big tangle and he knows this, so why is this a sad thought?