As American citizens enter the country, they should expect to present a valid passport and proof of smallpox, yellow fever and cholera immunization to the immigration officers.

Flew into Malabo on a US passport. The US is the only country that gives you visa free access to EG. (But see below). Some sites on the web suggest you need a smallpox and cholera vaccination as well as bank statements in addition to your US passport. My hosts assured me you needed none of that and they were right. I wasn't even asked to show my yellow fever shot. All you need is an immigration card the airline will give you.