US Republican presidential candidate Ben Carson speaks during a forum at the 2016 Kemp Forum on Expanding Opportunity in Columbia, South Carolina, January 9, 2016.[Photo/Agencies]

Ben Carson

Retired neurosurgeon Ben Carson is the only African-American candidate in both parties in this year's election cycle. He gained national attention in 2013 after his speech at the National Prayer breakfast, in which he spoke bluntly about healthcare.

Being a stark comparison to Donald Trump, the mild-speaking retired neurosurgeon once gained seemingly unstoppable momentum last August by supports from conservatives looking for Trump's alternative.

However, his polling number plummeted after the Paris terrorist attacks due to doubts about his foreign policy knowledge.