The Plague of Amwas lasted from 638 to 640 AD, in Syria, Egypt and Iraq. The plague was most severe in Amwas, Syria, so it became known as the Plague of Amwas. It is supposed to have caused the death of over 20,000 people.
The plague's symptoms are not on record. It could be that those who saw the symptoms of the plague did not live long enough to write them down, or to tell someone else, or if they did pass the information on that person did not live long enough to tell anyone. It seems as though the plague killed its victims quickly, it is said that men "died as by a sudden blow". This could imply there were no visible symptoms, and in the 600's people were not capable of doing the tests that we can today, which would expose internal symptoms.
Source:
http://answering-islam.org.uk/Books/Muir/Caliphate/chap21.htm
