Let me offer some context. Most passport holders in a given country are foreigners. Because when you are in foreign land, you need your passport, when you are home you typically don't. locals usually don't carry passports, especially in third world countries. It's the same even here to a certain extent. You are typically more likely to have more valid foreign passports than actual local passports in any given country. When that country is poor it's even more so. Second point. The country of origin does not stamp your passport. Only the country of destination. So if I fly from Liberia to Belgium with a congolese or a Burkina faso passports, both african countries by the way. You will not see a Liberian stamp in that passport because liberia is the country of origin. Instead it will be a belgium stamp. Now you ll ask, how did they get into Liberia without getting a stamp? Most movements into african countries occur by land borders, not air borders because aviation is not that advanced and that available. Well guess what they do not stamp your passport at land borders, in fact most borders in West-africa are unmanned. Trust me I know!
When they are maned, they just search you car for contraband, weapons, party affiliation, so they can execute you in the brush if you are from the wrong party. When no weapons, contraband and wrong party has been found, they wave you in. No id or passport checks because most indigenous locals don't have id, or passport. Most have never heard of either. So again, there is no way to know you came from Liberia if you don't buy tickets in a bundle but rather separately when leaving liberia. The people tackling those issues know this, which you don't. Which is why I keep making the expert point. Again this is tantamount to Hillary Clinton criticizing my wind calls.
So guess what, Liberians aren't the threat because most of them are unlikely to have a passport, let alone a valid one at this point. And applying for one now will not work because the country is in shambles. The problem is foreign nationals, chinese (Lots of them there), europeans, other Africans who pose a problem. The guy who brought Ebola to Nigeria was an AMERICAN citizen flying from liberia to Nigeria. Not someone from Liberia.
So you screening for Liberian passports or closing flights between Liberia and the US or West-Africa to the US will not stop this and will breed more issues.
Very last point. You can fit 3.07 United States inside Africa. So yes, it's a BIG problem! You are going to have to shut down a lot of flights, because Africans or infected foreign nationals will move by road from country to country to get out. And again borders from African countries to African countries are porous, not Mexico-USA border porous, but more like I am going to Walmart porous.