Pneumonia. She died in her sleep. She had been visibly suffering from a cold for a few days before and her mum (my Gran) got some meds from the GP the day she died. Just saying that, in our family experience, infants can go very quickly if signs are missed.
Gran never really got over it. Even in the throes of dementia some 50 years later she would wake up (like she did the day she found her baby in the cot beside her) and wail "the baby's dead, the baby's dead..."
Even now it breaks my heart to remember that.