As for Cassidy's heart condition, I have never had a problem with him dying because of it. The only way I am aware of to get him to die of it is to allow him any drugs or stimpaks. If he needs healing during combat, I make sure to have some of those healing powders you get in the beginning of the game handy. It's a real shame to have to use those because of the loss of perception really huts his shooting, but if he's got the shotgun equiped he has to get close to attack anyway so it's not real loss. Otherwise I heal with Doctor and First Aid outside of battle. Especailly do NOT give him Psycho, Jet, or Super Stimpaks, they seem to randomly kill him upon taking those. You can probably get away with a stimpak or two but I wouldn't try your luck, I'd just keep him in the back with a good rifle instead.