Gives a 20-point health boost, but not past 100 points. If you come across one of these but your health is only down to, say, 95, kill a few more bad guys and then grab it when your health is down into the low 80's. Once you grab your first adrenaline burst or acid vest, though, your health might never get low enough to use a med kit again!
Gives you only 1 unit of health BUT will increase your health no matter how high it already is. Rarely, a single derm patch may yield 5 or 10 points. When your health drops below 100, the best strategy on EASY and MEDIUM is to kill all the aliens first, pick up enough medical kits to bring you back up to 100, and then grab the patches for that over-100 boost. On HARD, though, this strategy just ain't worth it. When your health gets so high that you don't use med kits or even adrenaline bursts anymore, these one-pointers will still keep inching your health units up.
Boosts your health all the way up to 100 and gives you 30 seconds of complete invulnerability. Grab one and go kick some alien tail! For some reason, you will sometimes find this item on the "powerup" levels (all goodies, no aliens). Not sure why.
This one will pump you up to 200 health units. With one of these in the vicinity, you can take a lot of damage, then come back and grab it. If you're good, you will reach the point where your health stays so high, thanks to armor, that you can't even use these anymore.
Armor will NOT protect you from a facehugging (3 points) or a fall (5-15 points or death!). If you know a vest is nearby, you can fight enemies until your armor is low and then go back, get it, and come back fully armored. Or, if you are really low on ammo, you may prefer to use a lesser weapon (probably taking more damage) to dispatch enemies and then run back and grab the armor. If you are cautious, you can almost completely avoid losing any health points during the rest of the game once you've donned that very first acid vest. Three forms of armor are available:
This one gives 100 armor units. You can tell it apart from the body armor because it has a larger skull-and-crossbones and lacks "shoulders", but otherwise these two look pretty similar.
This one is a definite sight for sore eyes, but a lot less common than the acid vest. Two hundred armor units--enough said!
Allow you to run over alien carcasses or through acid pools without damage. These are time-limited (30 seconds). In places with acid pools you will want to clear your path of aliens first before donning the boots and racing to use them where they are really needed before they run out.