There are 4 levels of health: Healthy, Stable, Serious, and Critical, denoted by Health Dots. 4 Green is Healthy, 3 Light Green is Stable, 2 dots (1 Red, 1 Yellow) is Serious, and 1 Red is Critical. After Critical, your survivor is permanently dead.
Player-made safehouses provide a multiplier bonus to Fear and Fatigue healing (an apartment will reduce your F&F 1.2x faster than a NECRA safehouse). It takes 4 hours in a Safehouse to recover 1 Health Dot, though this can be reduced to 3 hours through safehouse upgrades .
My advice is when you hit Serious, it's time to make it to a nearby safehouse to recover.
I like to recover at Stable since it's an 8 hour downtime, compared to 12 hours if the survivors are at critical.
New tip: you can recover health while hiding still note, that Fear and Fatigue will not recover though.
If you've been sitting in a safehouse for a while and haven't seen your survivors recovering and are wondering why, it's because you must click the big yellow "Click to check recovery status" button to recalculate your survivors' current values; this will not happen automatically.
Infection If your character happens to get bitten, there is no cure. On this page you can see a picture of how to tell your survivor has been infected. When (and if) this character turns (3-48 hours I've been told) they will turn and wipe out their whole squad. Best to separate them from the squad.
Starting skills are important, and key. I think it's better to spend them on efficient skills than waste them on various Firearm skills.
Urban Forager, Adrenaline Rush, and Skull Crusher are the three I recommend.
Skull Crusher should be the first skill you ever put points into, this is because survivors must have skill in a weapon class before they will be able to do any damage with it. To survive you're going to have to fight off zombies and to fight zombies you're going to need points in weapon skills. Skull crusher is the best first choice in weapon skills because melee weapons don't require ammunition (which you have to scavenge).
Urban Forager, because it increases your percentage for loot. Every item type you loot gains you 2 Survivor Points (SP)
Adrenaline Rush, makes you move further. The less time you are in a high z-density zone, the less likely you are to get injured. Note, your squad moves at the rate of your slowest survivor. So try and keep them even. as one survivor with +50% will not move at that rate when everyone else is at +10%.
In the end, you will be acquiring these abilities anyway, so you might as well get them from the start when they can help you the most.
These are the only places you can recover Health, Fear and Fatigue. If at all possible, until you can create them (level 4), stick close to these, or at the very least mark them so you know which way to head.
Private Safehouses , when created, are surrounded by an hexagon. That is your Safehouse's area of effect and any location that can be scavenged within that hexagon has a huge Scavenging Bonus. In other words, better loot faster and the greater your SP gain from those successes.
Something to note. Anything you store in a Necra safehouse's public stash is open for anyone to take.
Save your ammo. It is one of the harder resources to find.
Don't push your squads, allow them time to recover. More so if you've spent SP on them, you want to protect your SP investment.
If you wipe out your survivors, don't worry, you'll keep any infocards you collected, though you'll lose any items you found. Depending on your spawn zone, you may wipe a few times (or even up to or surpassing 8 times) or keep your original squad until you hit the upper levels.
The other guides I linked are required reading IMO, the more you understand about how the game works the easier time you'll have.