Neonatal sepsis is a blood infection in infants under 90 days old. It can be early-onset within the first week of life due to infections passed from the mother during delivery, or late-onset between weeks 2-3 due to infections acquired after birth in the hospital. Symptoms include abnormal temperatures, breathing issues, vomiting, and lethargy. Diagnosis involves blood, spinal fluid, and urine cultures along with monitoring temperatures and blood work. Treatment consists of administering antibiotics under close nursing supervision and monitoring of the infant.