A mother who had measles as a child has a baby. If the baby is exposed to measles at 1 month old and does not get sick, what type of immunity does the baby have?

The correct answer is based on the concept of natural passive immunity. In this scenario, the mother, having had measles, has developed her own active immunity through exposure to the virus and subsequently forming antibodies. These antibodies can be transmitted to her baby during pregnancy through the placenta and through breastfeeding after birth. As a result, the baby is provided with these maternal antibodies, which grant temporary protection against measles and other pathogens without the baby itself having to actively fight the infection.

This type of immunity is termed "passive" because the baby relies on the mother's antibodies rather than producing its own in response to an infection. The immunity is considered "natural" since it occurs through biological means rather than through medical intervention, like vaccinations.

To clarify, natural active immunity would require the baby to become exposed to the measles virus and subsequently generate its own immune response, which is not the case here. Artificial passive immunity involves receiving antibodies through medical means, such as an injection, rather than through maternal transfer. Artificial active immunity, on the other hand, would involve vaccination, which also does not apply to the baby in this scenario.