Forget West Nile, how about Elkhorn, Wisconsin Virus:
Mosquitoes as vectors... It depends on how stable the virus (or, technically, virus-like therianthropic vector) is outside a regular host. Ever popular HIV is not very stable; it takes work to get it. Hepatitis B and C is a bit easier.
I'm sure that if werewolves revealed themselves, one of the first urban legends to circle around would be the person who got it from a mosquito bite or a dentist.
Type, Crossmatch, and Transform 2 units packed RBCs:
Blood collected from donors is routinely screened for HIV and Hepatitis, and it is submitted through a battery of basic tests. Given the sheer amount of genetic information that would be needed to make a virus-like agent, the lycanthrope virus would probably be at least platelet sized, possibly larger. It would thus show up, erroneously as thrombocytosis, though this is a common enough normal finding that it would probably pass if it wasn't too great. However, the blood would also show other weird lab results, such as in my storyline polycythemia (the opposite of anemia--too many red cells), an elevated amylase and lipase (normal levels in canines, that if screened for and found could be mistaken for pancreatitis, a serious inflammation of a digestive organ), and elevated cholesterol--around 300 or so. (Canines have higher cholesterol than humans, but have completely clean arteries--they don't form plaques the way we do.)
The blood might be rejected, and the werewolf donor might be advised to see a doctor about this or that, or even urged to go to the ER. (Coincidently, I'm in one right now--not as a patient, but as the physician.)
Are humans low in carbs?:
The general consensus is that new onset lycanthropy is a risk for sudden death, and the risks are greater the worse one's health is. Someone who is morbidly obese is at increased risk for all kinds of problems.
For example, remember a few paragraphs ago, when I mentioned in parentheses that canines don't have atherosclerosis, the hardening of the arteries?
(Canines have higher cholesterol than humans, but have completely clean arteries--they don't form plaques the way we do.)
If existing clots are broken down rapidly, these plaques can break loose and form emboli--clumps of clots and stuff that can get stuck in arteries, blocking them off. The first place they'll go is where the highest blood flow is, which happens to be some of the most vital organs--the heart, brain, and kidneys. If they plug up coronary arteries, it's a heart attack; if the brain, a stroke. If they get stuck in the kidneys, it's messy, but considering lycanthropic regeneration, there may be a chance of recovery. Alternatively, a clot can break loose in a large vein, travel through the right side of the heart, and lodge in the lung as a pulmonary embolus. Large ones of these can be at least as deadly as a well-placed stroke or heart attack.
Assuming what doesn't kill you makes you stronger, I figure the obese person will burn off enough weight to be able to move around a little better, but he or she won't instantly become an athlete. However, he or she will have a better chance to lose the rest of the weight, because of the accelerated metabolism.