Burkitt lymphoma, Epstein-Barr virus, T lymphocytes, cellular tropism, Child, Preschool, Cohort Studies, DNA, Viral, Epstein-Barr Virus Infections, Female, Herpesvirus 4, Human, Humans, Infant, Kenya, Male, Milk, Human, Prevalence, Saliva, Specimen Handling, T-Lymphocytes, Virus Latency