In a condition called Guillain-barré Sydrome(or GBS), there is antibody mediated damage to the schwann cell. The antibodies are like soldiers, which fight against intruders in the body. But in this condition(and various other autoimmune conditions), the soldiers start killing civilians instead of the intruders. This happens because the immune system incorrectly labels them as terrorist(very common in outside world also). So, the antibodies start attacking the schwann cells and result in damage to the myelin sheath around nerve fibres.
Now, since the myelin sheath is damaged, there is more exposed portion of the axon, and the internodal distance increases. I suggest you read this if you want to know how a myelinated neuron conducts signals faster as the myelin sheath acts as an insulator, which decreases the capacitance. The increased internodal distance delays the impusles at this point.
The damaged schwann cells will be eventually replaced by the process of healing, called remyelination. However remyelination does not revert back to original structure, but shorter internodes. Again, because of shorter internodes, nerve conduction is slowed down. Axonal injury may also be present along with demyelination and remyelination.
Sequential remyelination will cause layers of myelin being accumulated around the nerve cell, which gives it a characteristic onion peel appearence on cross section.