Seth Lane is 5 years old and has never lived a normal life. He suffers from a genetic condition called Severe Combined Immunodeficiency, or SCID, which means he has little to no immune system. It's also called "bubble boy" disease, since he lives in an isolated environment, away from the general public.

He's lived in a "bubble" at U.K. hospitals for most of his life; he tried to attend school for the first time last year, but got too sick. Now, if you want to visit Seth, you must scrub your hands and wear a plastic apron. You can't have any virus, even a cold sore. Everything has to be sterilized, including the air, and most things he touches get cleaned with scalding-hot water first.

Seth underwent one bone marrow transplant, but it failed. He now needs a second transplant, and his father, Nik, will be the donor. The process is risky and given Seth's health complications, there's no guarantee he'll make it through the next four months. But if it works, it could change his life forever.

To encourage Seth to keep fighting, his parents have asked people around the world to wear yellow to support him on March 27, and post photos online. They will print out the pictures and post them in Seth's hospital room. His mother is especially looking for creative and silly photos to make Seth smile. They'll also post a map of the world with pins for every country from which people have sent their support.

On their blog, Seth's parents say they started the campaign to raise awareness of SCID and "show Seth he is not alone in this." Hundreds of thousands of people have pledged to wear yellow so far.

Want to participate? On March 27, post a photo of yourself wearing yellow to Twitter, Facebook, or Instagram, and include the hashtags #WearYellowForSeth and #SCID. If you would like to support Seth and his family financially, a family friend has set up a GoFundMe page, and his parents encourage supporters to check out The Bubble Foundation UK, which raises money for medical equipment and other support for kids without immune systems.

Watch Seth make the plea himself, from his hospital bed:

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