I ate undercooked steak, what can happen to me?
The risks of eating undercooked steak vary based on the the region, the risk status of the person involved (are they a child, elderly, or immunocompromised in any way).

For normal healthy adults, the risks of eating a single bit of under cooked steak in any of the industrialized nations are fairly small.  


Nothing might happen
You might suffer from food-borne illness, most likely e coli. from steak, with consequences ranging from:

You might get a "tummy ache"
You might suffer flu-like symptoms or vomiting for a few days
You might die, but it is quite unlikely



Note that food borne illness normally takes at least 24-48 hours to develop, and is related to the quantity of contaminated food consumed.  One bite is lower risk than a whole meal (lower, not none).  

If you are seriously concerned, you should consult your physician.

See also:  Foodborne Diseases Factsheet from Texas state government