The young are most at risk from all infectious diseases including viruses but evolution has provided them with "maternal antibodies", inherited immunity against diseases the mother has been exposed to. (The elderly are also at risk because of failing immune systems.) Maternal antibodies are passed in the first milk that is suckled. However, puppies and kittens do not inherit the white blood cells that manufacture these antibodies, just the antibodies themselves. Inherited protection gradually diminishes, disappearing almost completely by, at the latest 12 weeks of age. Eventually, your pet will produce its own protection when its own white blood cells learn how to produce antibodies. They do so when exposed to infectious agents such as viruses, bacteria and fungi. They also do so when exposed to the modified infectious agents used in vaccines.