Complex sentence: One hypothesis to the initiation is that an infection stimmulates the innate immune system, which spreads viral epitopes along with myelin epitopes to T cells, which then are activated to "attack" the myelin cells.
Split into simpler sentences: One hypothesis to the initiation is that an infection stimulates the innate immune system. It spreads viral epitopes along with myelin epitopes to T cells, which then are activated to "attack" the myelin cells.