The man presented himself to the accident and emergency unit of Colchester General about 08:30 BST.
A spokesman said: "It is very, very, very, unlikely to be Ebola as he has not been to any of the countries where it is reported."
The hospital said he had been placed into isolation purely as a precaution.
Public Health England advised the hospital the man's "criteria" for having caught Ebola was "too low" for testing but added he should be tested for other diseases relating to time spent in African swamps.
The man, thought to be in his 50s, remains in isolation and has started a course of antibiotics where he is "stable and comfortable", said the hospital.
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(Includes one in the US and six in Mali)
4,809 Liberia
3,955 Sierra Leone
2,536 Guinea
8 Nigeria
A "feverish" man has been placed in isolation at a hospital in Essex as he undergoes a series of tests having returned to the UK from east Africa.