Is there a single word that means "cause(s) of death"?

Yes: killer.¹ For example, the following two sentences are nearly synonymous:

Heart disease is the biggest killer in the United States.
Heart disease is the biggest cause of death in the United States.

As with all synonymous terms, there are subtle differences of tone. Cause of death is more neutral or formal. Killer has a more forceful impact, as it is also used in contexts where it suggests murderer.²

Killer is a good word, as another answerer has suggested. If you like quaint Saxon words, as I do, then there is also the word bane. (However, you can get away with writing words like that only if your English is very fluent. The word bane probably has no place in journeyman-level international English, where killer is the better word. If a Japanese wrote bane to me, I should immediately wonder if he had not mistyped some other word. To gauge the matter, consider the unlikelihood that the word bane would occur in any English-language life-insurance policy of the 20th or 21st century, but rather killer or, regrettably more likely, cause of death.)

C.O.D. Not a word but a common abbreviation.