After an investigation, the deaths of two infants and the sickening of a third at Miami Children's Hospital is a medical mystery. The infants, born extremely premature, were in the hospital's Neonatal Intensive Care Unit when two of them died of a common yet lethal bacterium in March. County health investigators found 23 strains of the bacteria Pseudomonas aeruginosa in faucets, sinks, and drains throughout the unit, but said that none matched the strains that killed a 7-day-old boy and a 21-day-old girl.