What could cause chills?

Many things. Chills and rigor are synonyms and are seen due to the release and action of cytokines and prostaglandins as part of the immune response which also increase the body's set temperature in the hypothalamus. The person then feels cold in its environment and has chills. These are caused by influenza, tonsillitis, pneumonia, biliary sepsis, pyelonephritis, abscess, malaria, amphotericin b, and others.