Viral pneumonia has been ruled out as the disease that has killed over 20 infants in the last three weeks.
This came as the number of infants who have died from the unknown disease in Nakuru and parts of Baringo rose to 23 after three new cases were recorded.
The number of the confirmed cases also rose from 177 to 215 in the last one week, raising concern and fear among parents.
Tests done by the Kenya Medical Research Institute (Kemri) turned negative against viral pneumonia, causing more confusion and anxiety among medics and parents of the affected minors.
Nakuru Health Executive Mungai Kabii said officers from Kemri had moved to the county to further investigate the cases.
"The most affected areas are estates around Nakuru, though we have received a few cases from the neighbouring Baringo," he said.
Kabii noted culture samples sent to Kemri last week had turned negative.
"Kemri has confirmed that this is not viral pneumonia as we suspected and we have embarked on more tests," he said.
He added they were currently treating the disease as a 'sudden acute respiratory illness' whose cause was unknown.
The official said after the results from Kemri, it had become hard and challenging to deal with the disease as medics were not sure what they were dealing with.
"We have so far lost 23 infants to this disease and have 215 confirmed cases as we seek more information about this new outbreak," he said.
Kabii called on parents, whose minors were suffering from high fever and cough, to seek medical attention with immediate effect.