The first question is what your werewolf looks like. The traditional werewolf is a huge, powerful figure, covered in shaggy fur, with a distinctly canine facial structure: pointy teeth, long jaw, nose at the end of a snout, pointy ears, and so on. They might even have a tail, paw-like hands, or reversed dog-like 'knees'.
Almost all of that would be impossible, unless the soon-to-be-werewolf were to undergo substantial plastic surgery, which surely wouldn't be an overnight change. So, we'll have to make our werewolf a little differently. That isn't actually going to be hard; normal humans in little more than face paint can look so very different that people will attribute all sorts of terrible things to their features. Thus, the long face, pointy ears, tail, paws and reversed knees can be done away with.
The most obvious difference is going to be body hair. Our werewolf is going to need some serious hair growth. Two things will affect that: hair color, and hair length. Dark hair will show up much better than light; the darker the better. Second, hair length. For a furry coat, our werewolf will need at least an inch of hair on his arms and face. Other locations, like back, chest, legs, etc. would be helpful, but not absolutely required, especially in climates where clothing covers most of the body. Some people claim hair growth of over 3 inches in a month; pushing the human body to its limit could result in inch-long hair in a matter of hours, especially if supplementing existing hair. Even if it isn't an inch long, hair grown so quickly will be very unhealthy, twisted and matted, and will give the appearance of long hair.
Werewolves are tremendously powerful; they fight with their claws and teeth, even against powerful weapons like guns. They can take a lot of damage without going down.
This is actually the easiest part to build. When fueled by adrenaline, normal humans can do superhuman feats, like lifting cars or smashing through concrete walls. Drugs exist that can raise the strength of humans, and many more drugs exist that simply turn off limiting switches in our brains, letting humans use strength even when it does incredible damage.
The right concoction of pain reduction and adrenaline could make even a couch potato into a world-class athlete for a day. After that, of course, they would be bed-ridden for weeks, trying to heal the torn muscles and ligaments, but for a day, they would be monsters. And speaking of monsters...
Werewolves aren't known for their thinking ability. When they "turn", they go on murder-filled rampages, killing and maiming. This, too, is easy to replicate; just search for "bath salts zombie" and you'll have real-life examples of mind-altering drugs turning regular people into horrifying monsters.
Teeth & Claws
It's not that hard to get pointy teeth, but reverting after a night of murder will certainly be difficult. For this, we'll have to stick with the imagination of the masses, rather than real pointy teeth. Swollen jaw muscles can cause the jaw to hang open; in that case, some drooling can be expected. Blood from either an internal mouth injury, or from an outside source, can make the most pristine set of teeth look wildly nightmarish.
Claws, on the other hand, can be grown fairly quickly, especially if the body isn't growing them as it ought. The data I could find on fingernail growth suggests something like 3.5 mm per month, which is hardly claws; however, a rush of growth could turn otherwise normal fingernails into yellowed claws in a matter of hours. The fingernails would be weak, jagged, and horribly ugly, but sharp and nasty nonetheless.
Putting it all together
A bacteria invades a host's body. Over the course of a month, the bacteria causes the host's hair to grow darker and more quickly. Finally, after almost exactly 29 days, the bacteria causes pockets of chemicals in the body to rupture.
The chemicals have several effects. The first chemicals cause rapid hair and fingernail growth. The hair and nails are unhealthy, twisted, and ugly because of its rapid growth. Next, the bacteria floods the body with a concoction of chemicals that reduce pain, increase adrenaline to borderline deadly levels, and cause swelling throughout the body.
The infected person flies into a homicidal rage; blinded by drugs and pain, the host attacks anyone who comes near, unable to comminucate in anything beyond growls. The infected's arms and face are covered with coarse hair; their fingernails are jagged claws. They feel the extreme urge to bite and scratch, because that causes the sharp pain in their mouth and fingers to numb.
After their episode (usually only a few hours), the concoction of chemicals causes the excess hair to fall off. They will be weak, fevered, and tired; additionally, any injury they sustained will remain. They likely have no memory of the previous events.
Those that the scratch will become infected; since the disease takes roughly 29 days to mature, the exact time of maturation is roughly the time between full moons, leading to the myth that werewolves only come out at the full moon.