From Halopedia, the Halo wiki

There is more information available on this subject at Calcium on the English Wikipedia.

Calcium is a chemical element designated by the symbol Ca and atomic number 20. Calcium is a soft gray alkaline earth metal that is essential for living organisms. Calcium is necessary for the mineralization of teeth, bones and shells, nerve signaling, muscle contraction, the secretion of certain hormones and enzymes and the triggering of cellular functions. Most humans consume high-calcium foods as part of their standard diet. Calcium is also vital to the Flood, which requires high quantities of calcium from organic matter in order to develop an adaptable framework for Flood Super Cells to function.[1] Humans, Sangheili and Jiralhanae are all viable hosts for the Flood because of the calcium they possess, while Kig-yar, Unggoy and Yanme'e lack the necessary amounts to become sustainable forms.