What does the word "mortability" exactly mean?

It's a "nonce-word" (a "one-off" neologism that in this case may have been coined more than once and/or occasionally copied). Here it is in the 1928 publication Journal of State Medicine - Volume 36

It is important to distinguish between liability to attack — infectability — and liability to die as the result of that attack - mortability as I should like to term it

The word infectability is perfectly valid, but I doubt you'd find mortability in any dictionary. Clearly it's based on mortality, with the specific sense of (measurable) "death rate" / "chances of dying".

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Thanks to @rajah9 for pointing out that the "correct" term (as defined in the full Oxford English Dictionary) is...

morbidity 1b. Medicine
The rate of disease, or a specific disease, in a population; (emphasis mine)
= morbidity rate

And here's a Google Books link showing just how often the "equivalent" terms mortability and morbidity occur in close proximity within the same text (surprisingly often, I would say). One almost gets the impression many medical writers would like the former term to have greater currency. So they keep writing it (alongside the "correct, explanatory" word "morbidity"). But dictionary-makers keep ignoring them!