Viral shedding occurs by 7 days following infection. CDV is commonly spread by aerosol or droplet exposure; however, it can be isolated from most other body tissues and secretions, including urine. Transplacental infection can occur from viremic dams. Virus can be excreted up to 60 to 90 days after infection, although shorter periods of shedding are more typical. Contact among recently infected (subclinical or diseased) animals maintains the virus in a population, and a constant supply of puppies helps provide a susceptible population for infection. Although immunity to virulent canine distemper is prolonged or lifelong, it is not as absolute after vaccination. Dogs that do not receive periodic immunizations can lose their protection and become infected after stress, immunosuppression, or contact with diseased individuals. Many susceptible dogs can become subclinically infected but clear the virus from the body without showing signs of illness. Although most recovered dogs clear the virus completely, some may harbor virus in their CNS. The prevalence rate of spontaneous distemper in cosmopolitan dogs is greatest between 3 and 6 months of age, correlating with the loss of maternal-derived antibodies (MDAs) in puppies after weaning. In contrast, in susceptible, isolated populations of dogs, the disease is severe and widespread, affecting all ages.