Flags over the U.S. Capitol were flying at half-staff again this week as lawmakers for the fifth time in just eight months mourned the passing of one of their own.

The death of Rep. Jo Ann Davis, R-Va., over the Columbus Day weekend accentuated what has been a particularly grim year for Congress — made up of 535 senators and representatives.

Only one member, Rep. Robert Matsui, D-Calif., died in office during the 109th Congress running from January 2005 to January of this year. The 108th Congress passed without a single loss.

"It's somewhat cyclical. There are stretches when there aren't any deaths for a while," said Senate historian Donald Ritchie. There are also times of generational change, such as the 83rd Congress in 1953-54, when nine senators, many first elected during the New Deal era, succumbed.

Davis, who died of breast cancer, was only 57. Her death was preceded in February by Rep. Charles Norwood, R-Ga., who died of cancer and lung disease at age 65; in April by Rep. Juanita Millender-McDonald, D-Calif., a cancer victim at age 68; and in June by Sen. Craig Thomas, R-Wyo., who was 74 when he died after a fight with leukemia. Rep. Paul Gillmor, R-Ohio, 68, was found dead in his apartment in suburban Washington in September, apparently after falling down stairs.

Thomas was the first senator to die in office since Sen. Paul Wellstone, D-Minn., was killed in a plane crash in 2002.

Fred Beuttler, deputy historian for the House, said the four deaths in the House this year compare with an average 2.7 deaths for each two-year Congress over the past two decades.

Congress has naturally aged as Americans in general have come to lead longer lives. The average age of the 435 House members is now 56. The average senator is 62, the oldest average age in history. Members of the first Congress in 1789 were on average 47 years old.

Currently, one fourth-of the Senate's 100 members are in their 70s and 80s, including six octogenarians. They are led by Sen. Robert Byrd, D-W.Va., who will celebrate his 90th birthday next month. Byrd, while noticeably more frail now, still chairs the Appropriations Committee.

Two other senior senators, 80-year-old John Warner, R-Va., and 75-year-old Pete Domenici, R-N.M., have cited age and health in decisions not to run for re-election next fall.

The oldest member of the House, at 84, is Rep. Ralph Hall, R-Texas.

Gillmor's death, while resulting from an accident at home, also pointed out that being a member of Congress can be a dangerous business.

Some 60 members have died from other than natural causes while in office, nearly a third of those from air crashes. There have been at least 10 suicides and three who died in duels, according to a 2002 Congressional Research Service study.

Six were murdered, including Sen. Huey Long in 1935, Sen. Robert Kennedy in 1968 and Rep. Leo Ryan, who was shot before Rev. Jim Jones ordered the mass suicide of his followers in Jonestown, Guyana in 1978.

According to the CRS report, Rep. Cornelius Springer Hamilton was killed by an insane son in 1867, Rep. James Hinds was assassinated in 1868 and Rep. John McPherson Pinckney was assaulted and killed in 1905.

A seventh lawmaker, Sen. Edward Dickinson Baker, was killed in an 1861 Civil War battle.