A four-year-old boy in the northeastern US state of New Jersey died suddenly and without warning as a result of the outbreak of a respiratory virus, enterovirus D68, officials said.
Eli Waller, who lived in Hamilton Township, died on September 25 but his cause of death was only diagnosed at the weekend.
The youngest of triplets, he didn't go to school during the day and went to bed with red eyes. He never woke up.
"I think Eli's case is an exception to the other cases in the country," Jeffrey Plunkett, health officer in Hamilton Township, told ABC television.
"He had no sign of any illness that night and his passing was sudden and shocking."
Enterovirus D68 typically causes flu-like symptoms, but in some cases it can cause wheezing and breathing problems that may require hospitalization.
A spike in cases among patients nationwide has reached 538 people, most of them children, in 43 states, according to the Centers for Disease Control and Prevention.
The death of a little girl was announced last week in the state of Rhode Island as a result of an unusual combination of enterovirus D68 and a staph infection.
On Sunday evening, scores of worried parents met local officials at a gathering in Hamilton Township.
Experts are struggling to understand why so many young people -- ranging in age from one to 18 -- across the United States have fallen ill from the virus in the past two months.
There is no vaccine to prevent EV-D68, and frequent hand-washing is the best way to prevent it, experts say.
Enterovirus D68 is not new. It was first discovered in 1962, but has remained fairly uncommon, experts say.
Viruses in this family typically circulate in the late summer to early fall, before flu season begins in earnest.