Vitamin K is needed by humans for blood clotting. Older children and adults get most of their vitamin K from bacteria in the gut, and some from their diet. Without enough vitamin K, small cuts can go on bleeding for a very long time and big bruises can happen from small injuries. Bleeding can also occur in other parts of the body, such as the brain (causing one type of stroke).
Babies have very little vitamin K in their bodies at birth. Vitamin K does not cross the placenta to the developing baby, and the gut does not have any bacteria to make vitamin K before birth. After birth, there is little vitamin K in breast milk and breastfed babies can be low in vitamin K for several weeks until the normal gut bacteria start making it. Infant formula has added vitamin K, but even formula-fed babies have very low levels of vitamin K for several days.
With low levels of vitamin K, some babies can have very severe bleeding - sometimes into the brain, causing significant brain damage. This bleeding is called haemorrhagic disease of the newborn (HDN).