You can only be infected by a particular disease once at any given time
When a character is injured by a contaminated attack touches an item
smeared with diseased matter, or consumes disease-tainted food or
drink, he must make an immediate Fortitude saving throw. If he
succeeds, the disease has no effect—his immune system fought off the
infection. If he fails, he takes damage after an incubation period.
Once per day afterward, he must make a successful Fortitude saving
throw to avoid repeated damage. Two successful saving throws in a row
indicate that he has fought off the disease and recovers, taking no
- For the first part (getting infected) you probably already figured
out that if you succeed on the first one, you gotta succeed on all
the next attacks to prevent being infected, so for that part it does
- The incubation part is when the disease is building up in your
system, you either catch the disease or not, you can't have it twice
in your body (real life logic and no more info in the rules) so after the ''first'' incubation
period (1d3 days for Filth fever) (you take 1d3 Dex, 1d3 Con
damage) you now have the disease and must make a saving throw once
per day afterward, so ignore the other wounds sources of the infection,
you already have the disease and cannot recatch it (nor having it 2 times as bad) while already being
infected so no other incubation periods.
If your question is really "Do diseases stack?": if they are different diseases, then yes, they stack - as shown in the spell Remove Disease, which states:
Remove disease cures all diseases that the subject is suffering from.(...)
I could not find better RAW examples, I guess rules were intended as such.