very two/three week old abandoned kitten

Question
Last week we were given a kitten that had apparently been abandoned by it's mother and hadn't eaten for at least a few days. For several day it was eating just fine and seemed to become healthy enough that we thought it would be fine. It started walking and playing.Then two days ago in the morning things started to go bad. It became weaker and weaker to the point where now it just lies there not even able to lift it's head. It has a rattle in it's breathing and a vet we called said it may have caught a cold. Do you know of anything we might do to help the little guy?

Answer
Definitely get the kitten into a vet on an emergency basis!  I have to stress that there is no time to lose with a baby this little.  Even at just a couple weeks old, a kitten can be put on antibiotics.  This may be the baby's only hope.

Honestly, there are dozens of things possible here.  Congenital problems often surface around the 3-week mark.  It may be heart problems.  It may be a viral illness.  It may be aspiration pneumonia (if she inhaled formula, perhaps).  

But a vet is definitely the only choice, and the sooner, the better.  Please take my word for it - I have lost too many kittens before even realizing how seriously ill they were.  Being given these warning symptoms is certainly a blessing.