Professor Peter Berry, a paediatric pathologist, told Nottingham Crown Court that in some of the cases he could not exclude criminal activity, such as the deliberate obstruction of breathing or administering a non-therapeutic substance, but other explanations were more likely.
Ms Allitt, 24, has denied murdering four children in her care in 1991 and attempting to murder 11 other people.
The prosecution has claimed that Liam Taylor, 8 weeks old; Timothy Hardwick, 11; Becky Phillips, 9 weeks; and Claire Peck, 15 months, were all murdered while patients on the children's ward at Grantham and Kesteven General Hospital.
But Professor Berry, an expert on sudden infant death syndrome, said it was most likely that Liam died from a respiratory tract infection. Although other possibilities could not be excluded entirely, he could find no clue from notes of the post-mortem examination to suggest otherwise.
Timothy was a deaf and blind epileptic with cerebral palsy and children with such a condition could die unexpectedly, possibly from a relatively trivial breathing infection.
Becky, whom the prosecution claims was killed by a massive overdose of insulin, could have died from a rare heart condition. Claire's death was consistent with acute asthma.
The trial continues today.