From the article: "The cause of the malnutrition could be low caloric intake because the requirements for age were 120 kcal/kg/day and the patient received 75 kcal/kg/day. Also, the patient was given whole milk, with greater protein content, causing a greater solute load. Therefore, if the infant did not receive an adequate amount of fluids, dehydration would be the result, as in this case.... The presence of steatosis associated with malnutrition, edema and alterations that were found in all the organs appear to be secondary to kwashiorkor type malnutrition. In this case the histological image is completely compatible with this type of malnutrition in contrast to marasmus-type malnutrition in which the main characteristic is loss of muscle mass. In kwashiorkor type malnutrition there is important visceral involvement and lymph depletion that causes secondary immunosuppression."

I don't fully understand it but sounds like the malnutrition could have caused her to be immunosuppressed. I also wonder if the mom wasn't producing enough breast milk in the first few months before switching to cows milk which would exacerbate her already low nutritional intake.