The epidemiology of Madame Bovary

Towards the end of chapter six in Part II of Madame Bovary, Charles, Emma, and Monsieur Homais discuss the dangers that may await Leon in the city of Paris:

‘That is true,’ said Charles; ‘but I was thinking especially of illnesses—of typhoid fever, for example, that attacks students from the provinces.’

Emma shuddered.

‘Because of the change of regimen,’ continued the chemist, ‘and of the perturbation that results therefrom in the whole system. And then the water at Paris, don’t you know! The dishes at restaurants, all the spiced food, end by heating the blood, and are not worth, whatever people may say of them, a good soup.’

The greater susceptibility of people form the countryside to infectious diseases is mentioned in William H. McNeill's Plagues and Peoples:

in the nineteenth century, before inoculations became standard, draftees into the French army from the countryside suffered-sometimes seriously-from infectious disease to which their city-bred contemporaries were almost immune, having already been exposed. As a result, robust peasant sons had a far higher death rate in the army than did undernourished weaklings drafted from urban slums.