Other forms: infantries
Armies usually contain different divisions for different purposes. For example, soldiers on horseback belong to the Cavalry, those who work in communications are in the Signal Corps, and soldiers specially trained to fight on foot are members of the Infantry.
It seems somewhat strange to have the word "infant" in a word that refers to a fighting military unit, but the word infantry once referred to the foot soldiers who were too young or too inexperienced to qualify for the cavalry. The word was taken from the Latin word for a youth, infantem, and so evolved into infantry after working its way through the Spanish, Italian, and French, appearing in English in the late 16th Century.