The 92-year-old Senator Robert Byrd, the longest-serving senator in United States history, is seriously ill, according to his office.
Byrd has been hospitalized three times since 2009. If he were not to recover this time, or he decided to resign his office, his replacement would be named by West Virginia Governor Joe Manchin, a Democrat.
Byrd's current term expires on January 3, 2013. Under West Virginia state law on handling Senate vacancies, "if the vacancy occurs less than two years and six months before the end of the term, the Governor appoints someone to fill the unexpired term and there is no election". Otherwise, Manchin would appoint an interim replacement, and an special election would be held in November to determine who held the seat in 2011 and 2012.
In other words, we are within a week of the threshold established by West Virginia law. If a vacancy were to be declared on July 3rd or later, there would not be an election to replace Byrd until 2012. If it were to occur earlier, there could potentially be an election later this year, although there might be some ambiguities arising from precisely when and how the vacancy were declared.