Four air conditioners attached to the roof of the white Bonfils Blood Center bus in the Elizabeth Walmart parking lot hum against the afternoon heat. Inside the bus, it is cool. The crisp, antiseptic smell of disinfectant permeates the white interior, and four navy blue, reclined medical beds, two on each side, line the walls.

Potential donors, settled into padded bench seats near the door, tap away at tablets, while phlebotomist Mark Wilson stands in the narrow aisle running between the blood draw stations, waiting.

Mark's first draw of the day is Michelle Stuart of Singing Hills. Stuart is a regular, donating whole blood every 56 days, which is the maximum healthy interval between donations. In addition, she is also designated as a “baby donor.”

Baby donors' blood tests negative for a flu-like virus known as cytomegalovirus (CMV). Once exposed to CMV, the body retains antibodies to protect itself. In addition, the virus can linger in the infection-fighting white blood cells, and the disease can be fatal if transmitted to low-birthweight infants or expecting mothers.

Since most adults have been exposed to CMV, Stuart is an attractive blood donor, but Liz Lambert, communications specialist for Bonfils, emphasized the need for people with any blood type to donate.

Stuart's blood will go for testing with the rest of the blood donated, and once it passes a screening for disqualifying factors such as HIV and hepatitis, it will be available within 48 hours to a patient at one of the hospitals served by Bonfils. The blood has a shelf life of 42 days.

Bonfils is the largest provider of blood to Colorado hospitals. It operates community donor centers and hosts up to 10 daily mobile blood drives each day in communities like Elizabeth to keep up with the 3,000 units of blood needed weekly in Colorado.

According to Bonfils, around 4 percent of Coloradoans donate blood. Donors must be at least 18 years old, or 16 with parental consent. To book an appointment on the Bonfils Blood Center webpage, potential donors need to pass a brief preliminary screening covering basic health, lifestyle, and recent travel history outside the U.S. or Canada. Once at the drive location, they are screened with additional health questions.

A visit to a donation center or bus takes 45 minutes to an hour, depending on the wait time, but the actual process of drawing a unit of blood (500 milliliters or approximately one pint) takes around eight minutes. Regular donors such as Teri Gindro of Elizabeth, Wilson's second draw of the day, know the drill.

Gindro, like Stuart, has been donating blood for over 30 years, but Gindro is not only providing blood to someone in need, she is also managing a condition called hereditary hemochromatosis, a genetic condition that causes the body to accumulate excess iron.

“I can feel it when it's time to give blood,” Gindro says. “My ankles and wrists start getting sore.”

The excess iron accumulates in Gindro's heart, liver, and pancreas and is linked to cancer, heart arrhythmias and cirrhosis. By donating blood regularly, she reduces the iron in her system. Recently the FDA approved the use of blood donated by people with hemochromatosis because the genetic condition is not transmitted through transfusions.

The effort to collect blood is a 360-day-a-year process. By the end of the afternoon when the mobile donation center pulls out of the Walmart parking lot, Wilson and his fellow phlebotomist Cheryl Bonaguidi will have drawn 15,000 milliliters of blood from 30 Elbert County donors, less than 1 percent of the blood needed in Colorado for the day.

For those who missed the opportunity to donate, the mobile donation center will return to Walmart on Sept. 9.