|It's worse than that, he's dead Jim!|
The Apothecary's job is to tend to the wounded, and to milk the gene seed out of dying Marine's Progenoid glands. Nice work if you can get it.
The Apothecary costs 1 equipment card, otherwise he is a normal Bolter Marine (If he has other weapons, work out some reasonable attack dice values).
If a Marine is killed within 1 square (diagonally OK) of an Apothecary, roll a die. If the result is a 1, the Marine's life is saved. Otherwise the Marine dies, take the dead miniature and place it on the Marine Control Panel (the Apothecary has the gene seed). The Alien player gets no points for that kill. If the Apothecary is killed, give any miniatures on the Marine Control Panel to the Alien, they now get the points. The Apothecary cannot heal himself.
You will presumably keep your Apothecary beside your heavy weapons. He will either save the heavy weapon gene seed, or draw fire that would otherwise be directed at the heavy weapons. Naturally the Alien will try to kill your Apothecary with Encounter cards if he is loaded up with Gene Seed. He may not be the best use of an equipment card, but if he doesn't get killed he has a good chance of saving a marine and stealing 10 points from the Alien.