Fail to act. I think WHO had the authority but were just afraid to yell fire, until enough people had burned. The wording of their early on statement was something like "We don't think travel restrictions are appropriate". Two weeks later world travel had obviously nosedived, because nobody wanted to be on a plane.
I think the CDC has already looked at the procedures some. They sent Tommy Thompson to get presidential authorization for involuntary quarantines if necessary. And he got it. Although they did say they would only use it if people were unwilling to abide by voluntary quarantines.
The ACLU won't be able to obstruct CDC quarantines. They can fight it in court afterwards, but will be no more effective than fighting to free the terrorists. The CDC and this president will not back down just because of one ACLU suit. And the ACLU will probably think twice before initiating it, if there is not some substantial doubt about the threat. Otherwise they would lose and set a precedent.