Hand washing is the cheapest and easiest public health measure around. Washing (with soap) for twenty -- that's 20 -- seconds after using the loo could do wonders to prevent the spread of all sorts of diseases. Carry some waterless hand cleaner if you're likely to run into the situation where there's no water AND soap.

Hospital personnel in the US have cut iatrogenic (hospital-caused) infections more than in half simply by handwashing (soap & water or the waterless cleaners) before moving from one patient to another.

And don't hawk up a big 'un and spit, either. Back when TB was prevalent you could have been fined for public spitting -- a law we should perhaps renew. Use a disposable tissue -- wipe your work-sweaty brow with a handkerchief (or use it to discretely carry your smelling salts).