TNS 
TNS stands for Trapped Neutrophil Syndrome, an hereditary disease  where the bone marrow produces neutrophils (white cells) but is unable to effectively release them into the bloodstream. Affected puppies have an impaired  immune system and will eventually die from infections they
cannot  fight. Once thought to be rare, it is now believed that the disease goes  undiagnosed for several reasons.  First, not very many veterinarians know about  the disease to look for it.  Second, even when looking, blood counts do not always show lower than normal neutrophil (white blood cell) counts. Finally, because it is an autoimmune-deficiency disease, young puppies present a variety of symptoms depending upon what infections they fall prone to. Thus many cases  are not properly diagnosed and have just been thought to be "fading  puppies".
Making the diagnosis even more difficult is the fact that age of onset varies depending on which infection is involved at the time.  Most puppies become ill before leaving the breeder but some do not have symptoms until later. The oldest known survivor was 2 years 8 months. Most affected  puppies die or are euthanized by about 4 months of age. The research now suggests that the gene is widespread throughout the Border Collie  breed.
TNS cases have been positively diagnosed in Australia, Great  Britain, Hungary, the Netherlands , New Zealand , and the United States . TNS carriers have been identified in lines not related to the Australian and  New Zealand lines where  the disease was first identified - including 100% ISDS lines. It is  autosomal recessive, which means that both parents
have to be carriers to  produce an affected puppy. 
A DNA test is available through the University of New South Wales in Australia and Optigen for the United States and the UK.