Final answer:
Passive immunity is a short-lived immune protection passed from mothers to their babies through the placenta and breast milk, conferring immunity without the infant's immune system having to be activated.
Step-by-step explanation:
The short-lived type of immunity that mothers pass to their babies is called passive immunity. This naturally acquired passive immunity involves the transfer of antibodies from the mother to the fetus through the placenta, particularly IgG antibodies, which protect the newborn for the first few months of life. Additionally, secretory IgA antibodies are transferred to the infant through breast milk, offering protection against pathogens while the infant's own immune system is still developing.