A neurological disease is one that affects the brain or the system of nerves running throughout the body. The signs of illness can range from very mild (a weakness in one leg) to very severe (the inability to stand). In order for your vet to investigate the disease they need to know where the problem actually lies. If your pet has difficulty walking this may be because of a problem with the nerves in its leg, pressure on the nerves in its spine (like a slipped disc) or a problem in the brain. Only by careful examination can your vet identify where the problem is likely to be in order to perform the most appropriate tests.