Immunity occurs when a mother breastfeeds her baby.

During the last 3 months of pregnancy, antibodies from the mother are passed to her unborn baby through the placenta. Breast milk also contains antibodies, which means that babies who are breastfed have passive immunity for longer.

i think that is new experiments yield untrustworthy data and should be carefully considered, because you don't know what will happen with the experiments.

when it is at an intermediate population density