Hello, rigor is usually a sign of serious infection although sometimes viral diseases may cause it as well. Influenza is one of those diseases. Other common causes include urinary tract infections, pneumonia and cholangitis. Since he s been treated for a cold, a respiratory tract infection is the most likely cause. He should return to his doctor for an assessment. Diseases that cause rigor should be promptly diagnosed and treated. I hope I ve answered your question. Please let me know if you need further assistance. Kind Regards, Dr Panagiotis Zografakis, Internal Medicine Specialist