Posaconazole (MK-5592) Intravenous and Oral in Children (<2 Years) With Invasive Fungal Infection (MK-5592-127)

Inclusion Criteria: * Panel A: is undergoing treatment for possible, probable, or proven IFI known or suspected to be cause by fungal pathogens against which POS has demonstrated activity (which can include candidiasis) * Panel B: has an investigator-assessed diagnosis of possible, probable, or proven IFI known or suspected to be cause by fungal pathogens against which POS has demonstrated activity (and cannot include candidiasis) * Has a central line (eg, central venous catheter, peripherally-inserted central catheter) in place or planned to be in place before beginning IV study intervention. * Has a body weight of ≥500 g * The participant (or legally acceptable representative) has provided documented informed consent for the study. Exclusion Criteria * Has received POS within 30 days before Day 1 * Has cystic fibrosis, pulmonary sarcoidosis, aspergilloma, or allergic bronchopulmonary aspergillosis * Has a known hereditary problem of galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption * Has known or suspected active COVID-19 infection * Has a known hypersensitivity or other serious adverse reaction to any azole antifungal therapy, or to any other ingredient of the study intervention used * Has any known history of torsade de pointes, unstable cardiac arrhythmia or proarrhythmic conditions, a history of recent myocardial infarction, congenital or acquired QT interval (QT) prolongation, or cardiomyopathy in the context of cardiac failure within 90 days of first dose of study intervention * Has received any listed prohibited medications within the specified timeframes before the start of study intervention * Has a known hereditary problem of galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption (Part B) * Has suspected/proven invasive candidiasis (Part B) * Has enrolled previously in the current study and been discontinued * Has QTc prolongation at screening \>500 msec * Has significant liver dysfunction * Is hemodynamically unstable, exhibits hemodynamic compromise, or is not expected to survive at least 5 days