Unlike infants born with addiction, these children are coming across heroin and other drugs in the days and years after birth.

In Philadelphia this summer, a 9-month-old rolled onto a needle while in bed with her father. Kyleeh Isabella Mazaba, 20 months, died after drinking methadone left in a water bottle in the family van. James Lionel Vessell Jr., 2, swallowed oxycodone pills he found in a purse on his mother’s bed. And in early August, Kentucky officials treated an infant and three emergency responders believed to have been sickened by carfentanil-laced heroin that traveled through the air.

Often, emergency responders attempt to revive children with Narcan, an overdose reversal drug that works on small bodies as well as large ones.

Then come the questions for investigators. How did the substance get there? How did the child find it? Can this be stopped?

Sometimes officials charge caretakers with neglect or manslaughter. In one case this year, the authorities accused a couple of child endangerment after they admitted to rubbing Suboxone on their daughter’s gums, an attempt to hide the fact that she was born with an addiction.

But often the details go undiscovered, with witnesses too young to offer their own accounts.

Penny Mae Cormani was born Nov. 12, 2014, to Cassandra Leydsman and Casey Cormani, both mired in addiction. A year later, while the three of them were staying with another couple, Penny ingested enough heroin to kill a grown man.

Ms. Leydsman, the grandmother, said she believes Penny was scooting around on the floor during breakfast and found a bit of heroin, eating it like any child would. The official record is silent on exactly how the infant found the substance. But Penny’s parents pleaded guilty to third-degree felonies — Cassandra to child abuse homicide and Casey to attempted manslaughter — and went to prison.