The Smithsonian National Zoo's giant panda cub receives his first vaccine against the canine distemper virus on Thursday, Sept. 8, 2005. Zoo veterinarians say it is customary with mammals to wait until they are between the ages of 8 and 10 weeks to begin vaccinations, as this is when antibodies in the mother's milk are thought to begin declining and the cub's immune system should start strengthening on its own. The vaccination was part of the cub's fifth health exam. At the time of the exam, he weighed 7.48 pounds, measured 21.25 inches in length, could support himself on his front legs, and was trying to crawl.