BOSTON -- Siblings from Chelmsford are battling an immune-deficiency disease. The children have been sick since birth. They have to take medication just to fight off infection.

NewsCenter 5's Heather Unruh reported Thursday that when you look at 20-month-old Sasha Bush and her older brother, Will, playing, you would never know they are both battling a big disease.

"They've both been sick since birth," said Sasha's mom, Stefani Bush. "She had some type of infection in her kidneys which made her very ill -- pneumonia, ear infections."

"(Will) kept getting ear infection after ear infection," said Will's Dad, Ralph Bush.

Sasha and Will are sick three out of four weeks a month. They were born with primary immune deficiency disease, also known as PIDD. Doctors said PIDD weakens the immune system and can turn a simple cold into a serious infection.

"They may also develop other disorders, autoimmune diseases where their immune systems actually attack their own body. And some patients may have a higher rate of different kinds of cancer," said Dr. Francisco Bonilla, program director of clinical immunology at Children's Hospital Boston.

When Will was diagnosed, doctors told his parents that everything was off-limits.

"No playdates, no playgrounds, no day cares," Stefani Bush said.

According to the World Health Organization, there are 130 forms of PIDD. Doctors said Sasha's immune deficiency is worse than Will's. She receives weekly injections of a blood product to help fight off infections. All the supplies are in a tackle box.

"I'm going to get out her needle set," Stefani Bush said.

Stefani and Ralph Bush have been trained to give Sasha her treatments. Once the prep work has been done, the needles are injected below Sasha's back.

"We're going to put your backpack on. You're all done," Stefani Bush said.

The syringe pump goes in her backpack. She wears it for two hours, trying to lead a normal life.

The treatments are working, according to Sasha's parents. They said she has gotten sick less often. But her parents said blood donors are crucial to ensuring that Sasha and many others get the treatments they need.

"Blood just isn't needed for daily transfusion. It is needed for cancer patients," Stefani Bush said.