Byrd's spokesman Jesse Jacobs said the 92-year-old senator was initially admitted to a Washington-area hospital late last week for what doctors thought was a heat-related illness.
"However, upon further examination by his doctors, other conditions have developed which has resulted in his condition being described as 'serious,'" Jacobs said in a statement.
Byrd was a former member of the Ku Klux Klan white supremacist group.
The long-term West Virginia Senator also has the dubious distinction as the pork-barrel king of the Senate as reported here.
The senator's health has been failing for years, often forcing him to use a wheelchair. He was hospitalized last September after falling.
Byrd's illness could hurt Democrats' chances of getting the needed 60 votes in the 100-member Senate to clear a Republican procedural roadblock against the new financial regulation bill crafted last week by House and Senate negotiators.
Last week Democrats voiced confidence that the Senate would pass the 2,000 page finance bill that almost none of them have read. Byrd's illness could change that.
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