Sadly, Kevin Costner doesn't appear to have survived to deliver the 2020 census forms, so details are
on the sketchy side right now. But by our best guesses, between Infection and Collapse, Earth has managed
to rid itself of 99% of its pesky human inhabitants. Some cheerful math leaves us with about 70 million
left alive. Of course "alive" does count the Infected, who appear to out-number the less cranky of us by
a factor of about twenty-to-one. So 3.5 million quick-thinking or lucky-as-fuck homo sapiens sapiens vs.
66.5 66.6 (OoooOoOOoooOoo...) million homo iratus cruor...we are not the hometown favorites here.
True, we're crafty, have better impulse control, and can use guns. But the sad truth is, every second you take lining up the perfect shot/swing/stab at Zachary T. Infected, is a second during which you are *not looking behind you*, where 33.3 million *other* Infected are poking about for kidneys and twinkies.
So, the moral of the story is, going it solo is great for mobility, but sooner or later is going to give you one more Infected than you can handle at once (and that number may in fact be *one*) - do whatever you can when alone to *avoid triggering them*.
Ok, you did your best, but you set one off. Now what? The first step is critical. Take a single moment to soak in every sensation you're having, sight, sound, smell - this will be easier than it sounds, because hearing that Infected screech will open every adrenaline valve you've got and time will slow down a bit. Now your agile brain can put all that info to use. Are you hearing a single screech only? You may have a chance, jump to Mano-a-mano.
Otherwise, even if you can't see all of them, your brain has a good idea which direction they're all in, and will instinctively plot an escape course. That may be good enough - they're fast, but they're not the Flash...if you've lived this long, you might be able to out-run them for a while, until you can even the odds somehow (see: kite or die). But you can derail that instinct if there's something better at hand (that soaked-up moment of your surroundings will be invaluable here). Something strong that you can latch, without other entrances - a propped-open fire door, a car trunk, whatever - get in there, button it up and hold your breath. If we're right about how they act, they'll give up after 5-10 minutes of pounding on it and wander away. What, you don't know how to get out of a locked car trunk? *sigh* Try the glow-in-the-dark release handle standard on all cars with trunks made after 2015, or failing that, pushing/cutting through the seats into the backseat.
Since you're by yourself, you have one advantage - you have absolutely *no need to flap your lips*. As instinctive or satisfying as it may be to give power ki's with all your blows or quip one-liners after a headshot, remember - once your opponent stops screeching, the only thing that will bring more Infected over is *you being retarded*. In fact, this is good advice even before you're in this situation. I know how it can get when you're alone for long stretches - it's hard to resist the temptation to talk to yourself, or sing, whatever. Resist it anyway. Nothing sets these guys off like *your singing*. It's like Woodsy the Owl says: "Give a hoot - shut the fuck up or Infected will kill you."
So, it's just you and him (at least, for the moment - you hope). Remember, all he wants to do is kill you as quickly as possible. All *you* want to do is *NOT LET HIM*. This gives you more flexibility in attacks - he's going to come straight in swinging - if you can jump to the side before he changes course, you can get in a quick hit to the leg or knee. A break there and it's advantage: you...you can withdraw at an energy-preserving jog, or range him to death with whatever's handy.
If this isn't working, he's unusually fit, or you're, uh, not. So, a headshot is an instant-kill, sure, but only if you pierce the skull, and skulls are harder than you might think - remember, Infected are *NOT ZOMBIES*...they are alive, and if you poke large holes in their chest, they will fall down and die just like you (only with less whining). Piercing reach weapons are the order of the day here - spears, boathooks, pitchforks (watch out though, they can get stuck, which is awkward to say the least if you get adds), or guns - the easiest thing to hit on a human - the torso - is where you want to aim.
If you don't have a weapon advantage, you still have a skill advantage. Infected like swinging clubs or punching - maybe it's a cave-man throwback - but they never dual-wield and rarely pick up long weapons. So, repetez! Parry, riposte! Parry, riposte! They only have one combat style - charge in swinging. More of your brain is working, so use it. When you block a swing with your weapon (parry), try to do it from an angle so you can turn the same motion into an attack (reposte!). They almost never parry themselves, except by accident, so just keep parrying until it feels right, then strike!
You've got more than one on your heels. Fuck, fuck, fuckety-fuck. Things to keep in mind: they are single-minded predators, but not pack animals. So they're just going to keep trying to take the most direct route towards you. That means in an open space, unless you're faster *and* can out-stamina them (good luck), they're going to get you. But they don't use group tactics - if you can keep a sense of how far behind you they are from the sound, you can time your turns and get them to interfere with each others movement, or get caught up on obstacles. They don't plan ahead for navigation - with a good corner, you may be able to get an ambush-shot off, then vamoose again. They don't think really, they don't remember - if you find a trick that works, you can keep using it until they're dead - they won't "catch on".
If there's something large enough around, that can't be reasonably climbed on, you can make the parry-riposte cycle last a *lot* longer, since you don't have to worry about your rear. Though this does essentially eliminate your mobility, so I would make this choice only if you've no chance of maneuvering well anyway due to lack of fitness/injury/whatever.