Pus is a fluid that is created as a result of certain inflammations in vertebrates. In vertebrates, White blood cells fight the inflammation. Neutrophils destroy the agents that cause the infection, for example bacteria, Macrophages engulf and neutralise these agents. Pus is made of dead neutrophils and macrophages, mostly. In most cases, pus is the result of bacterial infection, but it can also result from other conditions, such as Psoriasis.