Nickolas Coke, of Colorado, was born with only a brain stem, a condition known as anencephaly (an-en-SEF-uh-lee).
The National Center for Biotechnology Information says most babies born with anencephaly live only a few days.
The centre says it occurs in 1 out of 10,000 births. Nickolas' grandmother, Sherri Kohut, tells KOAA-TV that she believes Nickolas came down with a virus.
She says he had difficulty breathing Wednesday and eventually stopped.
She says three attempts to revive him failed. Relatives say Nickolas survived without special medical equipment but required several kinds of medicine.
Kohut says Nickolas taught his family how to love and about the strength of family.
She recalls him laughing as he played among pumpkins recently.