Rabies question...?

Vets say that if a dog bit someone (and you're not sure if the dog has rabies or not), it should be quarantined and observed for 10 days. If it becomes sick or ill, then it could be rabid.

My question is: Why would a dog with rabies become ill within days after biting someone? What's happening to it?

Because that is the time the disease takes to develop and become evident

it doesnt mean that the dog gets sick from the person who he bit, it just means that the dog will show signs within 10 days of becoming rabid. most dogs change thier behaviour (as in biting when it was a non biter) if they are affected with an illness. if your dog or whose dog it was is under a vets care and has all its shots up to date there really isnt anything to worry about, but since the quarantine is mandated by law it has to be done, for the protection of the person and also the dog. Best bet is to have the animals shots up to date so that even if quaranteed there wont be any reason to have to put the animal down.

rapies virus is a slowly growing virus a man bitten by a rapied dog may have a latent period one year
the dog may contain the virus and do not show any symptoms or signs for up to 10 days

Aggressive behavior in a dog is a symptom of rabies, and by that, I mean if the dog has bitten someone, it is already probably exhibiting illness. Once symptom onset occurs, progression of the disease will be rapid.10 day quarantine is enough time to show whether the dog is either rabid of just plain mean.

Yes, there is a long latent period in humans AND animals infected with rabies. The virus can hide for months before onset of disease. The rabies virus attacks the central nervous system. This explains the altered behavior (including aggression, paranoia and a tell-tale fear of water), encephalitis and raging fevers.