There is a [When should I let my teammates die?](https://gaming.stackexchange.com/questions/270888/when-should-i-let-my-teammates-die) question, but mine is about a completely different issue.

When playing Mercy, I often see that my healing is not enough to heal all of the damage done, contrary to my habits from most MMO games. Mercy also requires to be near her teammates to heal them, putting me at risk sometimes if I decide to come closer.

So, how do I prioritize my teammates? Who should I heal first, and who can wait a bit, even if this waiting time can end up killing said teammate? 

When should I follow to heal my teammate, and when should I not? 

If I am healing someone who I manage to keep at some decent health, but someone else is dying just near me, should I try to help the dying mate?