Days 7 and 14 were chosen because CD4 T cell responses against B. pertussis develop rather slowly with a peak in lungs at around 14 dpc (7, 10), compared with the CD4 T cell responses to other bacterial infections such as Listeria monocytogenes or Salmonella, which peak around 7 dpc or earlier (27, 28). The gene discussed is CD4; the disease is bacterial infectious disease.