Complex sentence: The incidence of meningococcal disease in the United States is highest among infants younger than 1-year-old whose immune system is relatively immature and with a second peak of incidence in young adults, congregating closely, living in dormitories and or smoking.
Split into simpler sentences: The incidence of meningococcal disease is highest among infants (children younger than 1-year-old) whose immune system is relatively immature. In industrialized countries there is a second peak of incidence in young adults, who are congregating closely, living in dormitories or smoking.