To talk about life in general, life is considered a mass noun. That is to say, to talk about life as a concept, we say life in general; In general, life is short. This very usage is what makes life here a mass noun. Compare Death is inevitable.
However, we can also talk about lives in general. This talks about multiple instances of the singlular count noun life. For instance, we might say, In war, lives are cheap. Here the stress is on individual lives. But you could also In war, life is cheap, with life as a concept.
You could also say, in general, deaths are inevitable, in the medical profession
For 'types' of mass nouns, see this resource by the University of Washington.