"The susceptibility of ﬁsh to disease is partly dependent
on their environment, in particular on water temperature.
It is generally accepted that lower temperatures adversely
affect speciﬁc immune responses mediated by T helper
cells. The probable mechanisms involved in such
suppression in teleost ﬁsh are reviewed. Furthermore, the
effects of temperature on nonspeciﬁc defences, such as
phagocytosis and cytotoxicity, are described and total
immune competence in teleosts at low environmental
temperatures is discussed"
Here is ONE article
.. it's one of many. There are others here who can tell you more in detail, such as indjo or Oldfishlady.
Keep in mind room temp isn't what the temp of the water is.. your room may be set at 80, but the temp in the water is in the mid 70s, etc.
And that is exactly what I am saying.. genes and the environment they are being raised in plays a big role in how healthy a betta is during it's life span.