They are designed to create conditions similar to the mother's womb, where the body system of the baby, such as breathing, temperature control, and resistance to infection are not fully developed.
Incubators vary in design, but all have the same essential mechanism.
The temperature, humidity and oxygen controls all help the baby to compensate for poorly developed respiratory and temperature control, and allows life to continue on schedule as if it were in the womb. This still leaves the problem of feeding and resistance to disease, however. The controlled sterilized compartment of the incubator copes with infectious disease, because the only access to the baby is through the portholes in the side of the transparent bubble which covers the top to allow an all-around view of the baby. Handling is kept to a minimum and the baby is removed from the incubator as little as possible. Nursing care is usually given at, or around, feeding time, to minimize the danger of infection and keep oxygen levels even.