I guess it depends on how you define altruistic. To play devil's advocate, if you define altruism as doing something that has more concrete, tangible benefits to another than it does to yourself, then yes, altruism exists. If you define altruism as doing something entirely to benefit someone else, with no regard to any positive effect on yourself, I would argue altruism is mythical. Even one who sacrifices themselves to help another, such as trying to save a drowning stranger, does so because for them, the risk to their life is worth the opportunity to help. At the moment the decision is made, the person trying to save the drowning stranger decides that they would rather take the risk of their own death to try to prevent the death of the stranger, and that the negative consequences for them would be greater in their mind if they stood by and did nothing. Sure, the personal tangible benefits of not risking their lives is much greater than the personal tangible benefits of trying to save the stranger, but the internal costs of doing nothing are greater than the risk of doing something.
This brings up the question of why does something have to be "altruistic" to be virtuous? Isn't feeling good about helping others a virtue? Aren't there plenty of people who, judging by their actions, really don't care if bad things happen to others? I would argue, though, that the actions of those who want to help others are never truly altruistic, meaning done only for the benefit of another without any regard to their own benefit. And, there is absolutely nothing wrong with helping others because you gain something from it; in fact having the mindset that helping others is a better use of your time than simply doing other things you enjoy is truly a virtue. I would simply argue it is not a completely selfless action, and the fact that one chooses voluntarily to do so proves that the internal benefits of helping and internal costs of not helping tip the balance toward the supposedly altruistic action.
Not sure if I was able to write this as clearly as I thought it, so if clarification is required feel free to ask.