The immune system is the ticket here.  If it's in good order and working well, disease is avoided.  And it's always been that way.  I've found that loving pet owners often go out and buy isolated specialized products for the immune system- but that's not the best way to go.  Our pets, both our dogs and cats, need all the tools for good health and that's why they need a complete and balanced supplement. How the supplement is put together is very important. For instance, VITAL VITIES  include 400 IU of active Vitamin D3 for dogs and 200 IU  for cats in each daily dose .  VITAMIN D IS THE MOST IMPORTANT VITAMIN TO MAINTAIN A HEALTHY IMMUNE SYSTEM.  All the newest research shows how incredibly important this vitamin is (it's actually a hormone!) for a healthy immune system.   VITAL VITIES by Deserving Pets also has extra vitamin C. Even though dogs and cats make their own Vitamin C... well... in this day and age they need a bit more to help them eliminate toxic chemicals from their systems and boost their immune system.   Every vitamin and mineral and specially chosen whole food super-foods in VITAL VITIES work together to create optimum health.   Inner health prevents disease.