Of what illness were some of the pirates suffering?

A good number of the men are suffering from malaria.

That comes--as you call it--of being arrant asses," retorted the doctor, "and not having sense enough to know honest air from poison, and the dry land from a vile, pestiferous slough. I think it most probable-- though of course it's only an opinion--that you'll all have the deuce to pay before you get that malaria out of your systems.