Bacteria and viruses can be transmitted through breast milk, which is why the FDA views breast pumps, except for the hospital-grade variety, as one-woman devices [source: FDA]. Hygienic rental pumps and some newer personal-use pumps are designed differently and have special barriers and filters to prevent milk from entering the motor and tubing, which could lead to cross-contamination and mold growth. Every woman who uses or rents a hospital-grade pump is required to buy a new accessory kit -- which includes the tubing, breastshields and containers that may come into contact with her milk. This is considered a closed system.
Open systems have no barrier, allowing milk to come into contact with the motor or tubing. Depending on the make and model, milk can sometimes overflow into tubing and the diaphragm may become compromised. Because some of these diaphragms cannot be removed or sanitized, if a mother shares or reuses an open-system pump, she runs the risk of exposing her milk to another woman's milk every time she turns the motor on [source: Morbacher]. Before each use, all pumps must be properly disassembled and cleaned according to manufacturer instructions.
Like many issues regarding parenting, breast pumping has its critics. Some people believe breastfeeding has an advantage over pumping because it creates an emotional bond between mother and the nursing child [source: Lepore]. Also, there is the potential that pumping may cause nipple or breast damage if a woman fails to use a proper-fitting flange (allowing the nipple to be pulled into the tunnel without any friction).