There is some level of quarantine - the liberian government blockaded in the slum area where a clinic was raided, and sealed off about 50,000 people. That's probably about the limit of their efforts.
But frankly, you're also ignoring what's been actually happening: such an ambassador catching Ebola in one country, knowingly returning to another (via air travel), then staying in a hotel without telling anyone being treated in secret by a doctor, who also doesn't tell anyone, goes home, and spreads it to his wife.
Queue a couple 1000 potentially infected, by a vector which would be utterly missed due to plain old corruption and idiocy.
So what do you do now? Expand the quarantine? Do you even have enough troops to do that? And in the meantime, what about all the regular issues such as food and water, sanitation and normal completely curable diseases which will take hold.