Eli Waller went to sleep one night late last month and never woke up.

The 4-year-old from Hamilton, N.J., stayed home from preschool that day, when his parents thought he might have pink eye and didn’t want him to infect his classmates. He had no other symptoms of the respiratory virus sweeping the country — no fever, no coughing or sneezing, no muscle aches and no problems breathing. But sometime after they tucked him into bed Sept. 24 and before they tried to wake him the next morning, he died.

Eli’s death is believed to be the first in the United States directly linked to Enterovirus 68, or EV-D68, health officials ruled Oct. 3. Since August, the virus has hit nearly 600 people, most of them children, across 43 states and District of Columbia, according to the Centers for Disease Control and Prevention. Four others who died were infected with the disease, but it’s not clear what role it played in their deaths. Eli’s pink eye was unrelated.

“I think Eli’s case is the exception to the other cases around the country,” Hamilton Township health officer Jeff Plunkett told ABC News. “He had no signs of any illness that night, and his passing was sudden and shocking.”

Last month, several children in Colorado, including some who tested positive for the illness, also experienced neurological symptoms such as muscle weakness and paralysis.

Eli, the youngest of triplets, was “smaller and lighter than his sisters,” his father, Andy Waller, said in a statement. Although some children are more susceptible to the virus because of preexisting conditions, the medical examiner said this was not the case with Eli. The medical examiner determined Eli had brain and lymph node swelling that were a result of the disease, according to news reports.

On Sunday, Andy Waller released a tribute to his son. Here’s an excerpt:

My words probably won’t capture him well, but everyone who met Eli knows how he made people feel; imagine a shy little puppy who wants only to make people proud and happy, maybe tripping a bit over his own paws, but truly full of unconditional love. He was a beautiful mix of eagerness and hesitancy, need and striving, caution and surprise, all of which were grounded in a pure, unconditional love.

The youngest of a set of triplets, born much smaller and lighter than his sisters, Eli nevertheless persevered through all the difficulties that came his way. Eli was not the type to give up, and even though things never really came easily to him, he would just plug away, day after day, practicing sounds, or movements, or skills, until he would eventually get them. He did this entirely in an effort to make his Mom and Dad proud, and we can unequivocally say that we were, and will continue to be, so very proud of our little Eli.

Another child at Eli’s school, Yardville Elementary School in Trenton, N.J., was being tested Monday for a possible Enterovirus 68 infection. But Plunkett said its unlikely for a child to pick up the virus from surfaces.

“It’s possible surface contact, but I think that’s remote how the teachers and the custodial staff handle that classroom and that school, but nothing has been eliminated,” he told CBS New York.

School officials have disinfected all classrooms, water fountains and buses.

“We have procedures in place to make sure kids wash their hands before lunch, after lunch,” school superintendent James Parla told the station.

Enterovirus 68 was first identified in 1962. Most people who catch it get only a runny nose and a low-grade fever.

“There is no specific treatment for people with respiratory illness caused by EV-D68,” according to the CDC’s Web site. In addition: “There are no antiviral medications currently available for people who become infected with EV-D68.”