I'm neither an obstetrician nor a researcher in that field. But if I had to guess, this Wikipedia paragraph sumarizes things nicely regarding infection: "Passive immunity is also provided through colostrum and breast milk, which contain IgA antibodies that are transferred to the gut of the infant, providing local protection against disease causing bacteria and viruses until the newborn can synthesize its own antibodies." https://en.wikipedia.org/wiki/Passive_immunity

I'd have to read up to even speculate about the SIDS risk.