Medics IRL:
They're trained like everybody, but take an additional 4 month medic course. They carry many different kinds of treatmentment stuff (anything from bandages to infusions and ampules to give back liquids to those who lost too much blood). They're taught to NOT treat anybody until any immidiate threat is handled (in case of taking a hill - securing the hilltop, in case of a house - clearing the house). They will almost always save the life of an already incapacitated soldier rather than help him keep on fighting, as if you have a fragment/bullet in your arm/leg and you stop the bleeding, you would be unable to move the arm since no blood flows to it (and if you don't recieve medivac soon enough you will lose your arm/leg). Other stuff they do are meant to help an unconcious guy breath or give more liquids to a man who lost too much blood.
Implementation in INF:
First, you must understand you can hardly implement a bit of the medic's role (they take a 4 month, almost completely theoretical course (unlike combat training which is mostly practical training)) in combat and that the medics basically do their job after they finish dealing with any nearby threats rather than in mid-battle. The meaning of this is that it can mostly be used to save lives for the sole purpose of raising your score, when determining the winner by casaulties in a tied EAS match.
Bottom line is, it's very hard to implement and hardly has any effect on the game, and on some gametypes will have 0 effect.
If you want to implement it, make it so it doesn't count as a casaulty, or counts as 1/2 a casaulty if you treat some1 to survive the end of the round AND win it. Everybody can take basic medical gear (in the IDF everybody who has them carries 1-2 bandages and a tourniquet), and medics would have to give up stuff like extra grenades and magazines in order to carry the heavier medical gear like infusions+ampules, and more than just a single tourniquet and bandage...