Some runts are the smallest puppies of a litter but are still of normal size for their breed. These runts usually have no more health problems than any other puppy of their breed. A markedly undersized puppy is more likely to have health issues, such as heart problems.
Choosing a healthy puppy matters more than the puppy's size. Healthy puppies should be energetic and bright-eyed with no crusting or discharge around the eyes, nose, genitals or ears. They should walk and run without limping or stiffness and should have attractive coats that don't look greasy, dry or patchy. A puppy with normal hearing will respond to hands being clapped behind its head, and a pup with normal vision will respond to a ball being rolled within its field of vision.