My Cousin Was Feeling Sick For Weeks (just Generally Bad)

My cousin was feeling sick for weeks (just generally bad) so my aunt and uncle took him to the doctor. They kept taking him back after a couple days because nothing was changing even though they claimed that nothing was wrong. A few days later he passed out and wouldn't wake up so they took him to the nearest a children's hospital. At the hospital they put him on life support but had to take him off that night because soon he would be brain dead. He died last November and they still don't know what happened? Do you have any ideas as to what might have happened?