The current invention discloses methods to treat disease caused by virus infection, bacterial infection, parasites infection, autoimmune disease, disease caused by production of unwanted antibodies, sepsis as well as methods to treat cancer and methods for virus infection detection using blood purification method. The current invention provides a method to treat pathogen infection by inactivating the pathogens in the blood. During the treatment, blood is withdrawn from a patient and is separated into its plasma and cellular components. The plasma portion is treated with physical means such as UV radiation to inactivate the pathogens inside and then is returned to the patient. The current invention also provide a method to treat cancer especially to prevent tumor metastasis and tumor recurrence by removing and/or inactivating (e.g. killing) the circulating tumor cells (CTC) in the blood after removing the tumor or treating the tumor with therapeutical means.