True altruism is only possible in very limited circumstances; a person walking through a part of town he doesn't know throwing themselves in front of a car to push a child they do not know and are not related to to safety and then being struck by the car and instantly killed, all in such a short period of time it is a reflex and no real concious thought about it takes place is arguably altruistic.
Maybe this guy didn't have time to feel good about saving his friends, maybe he did; either way he's a hero;
http://iht.com/articles/ap/2006/10/13/america/NA_GEN_US_Iraq_Navy_SEAL_Killed.php
(So much for someone throwing themselves on a grenade to save their comrades never happening)
There is no 'pay-off' in terms of gene transferance as there is (for example) in social insects dying to protect another insects offspring.
However, as we are thinking creatures, most actions are non-altruistic as we get the benefit of feeling good about doing what we perceive as good. That is enough to make it non-altrusitic.
WHY we perceive such actions as good even if they are disadvantagous to ourselves is another question. My own thoughts are that comunities where people had these characteristics (even if only by means of an extended phenotype, i.e. a culturally transmitted way of acting) faired better than those where it was 'every man for themselves', as the 'penalty' for altruism was normally offset by the benefits, in in 'every man for themselves' societies there were no such beenfits.
Examples exist elsewhere in nature; Wolves and Cape Hunting Dogs feed nursing mothers and young they are not related too, the sick too. As already noted, Vampire bats who fail to feed for three days die; they remember the bats who feed them when they are in need and feed them in turn.