A maternity nurse for the NHS murdered seven babies and attempted to murder 10 more, even attempting to finish off some of those who survived, Manchester Crown Court heard.
32-year-old Lucy Letby is accused of carrying out a year-long murder spree where she targeted sick and premature newborns at the Countess of Chester Hospital in Cheshire between 2015 and 2016.
Manchester Crown Court heard how five boys and two girls died and another five girls and five boys "seriously collapsed" before being saved by nurses and doctors.
Prosecutors told the court how Letby returned to two of the babies that survived her initial attack and attempted to end their lives.
Letby allegedly poisoned two of her victims with high doses of insulin just days after they were born.
The methods the defendant allegedly used on the other victims is expected to be detailed as the trial moves on.
A criminal investigation was launched after concerns were raised after the number of deaths at the hospital's neonatal ward suddenly doubled within a year and a number of independent investigations failed to find an explanation.
Prosecutors told the court: "Doctors eventually noticed that the inexplicable collapses and deaths did have one common denominator.
"The presence of one of the neo-natal nurses.
"And that nurse was Lucy Letby."
Lucy Letby denies seven charges of murder and 15 of attempted murder.
For legal reasons, the media cannot name the 17 alleged victims and they will be referred to as Babies A to Q.
Nick Johnson KC opened the prosecution case by saying that consultants began raising concerns after a number of babies died after "deteriorating unexpectedly".
Mr Johnson said: "Not only that, when babies seriously collapsed they did not respond to appropriate and timely resuscitation.
"Some of the babies who did not die collapsed dramatically but then - equally dramatically - recovered.
"Their collapse and recovery defied the normal experience of treating doctors. "Babies who had not been unstable at all suddenly deteriorated. Sometimes a baby who had been sick but then been on the mend suddenly deteriorated for no apparent reason.
The trial continues.