You could also think of it as two different vectors or two different types of disease, each with its own attendant issues and likely solutions. I'm no epidemiologist but I am guessing that not all disease is created equal, and that the disease that flourishes due to filth may not be identical to the disease that flourishes due to overpopulation and close-quarters, etc.
In other words, these tags mean things in the fiction, and they result from specific choices (a larger population vs. a filthy population), so why not take the time to think about some specific possible consequences, rather than keeping it so abstract that when the time comes it's just 'disease plus ultra +++!!' without a clear connection to the problems of the holding.