Some cattle viruses can cross the placenta of the cow and infect the calf fetus, thus contaminating FBS and potentially making it unsuitable for use in cell culture. Some of these cattle viruses have a limited distribution and only exist in certain parts of the world. These are viruses of concern when importing FBS from infected countries – See Table of Diseases of Importation Concern for FBS. Other cattle viruses exist in all countries, and regardless of country of origin, the FBS must be tested or treated to assure freedom from these and other viruses (See USDA 9 CFR 113.53 and EMEA-CPMP-BWP-1793-02). The treatment of choice for most FBS is gamma irradiation at 25-45 kGy to guarantee freedom from viruses.