The first time a patient experiences KLS, it usually occurs along with symptoms that are similar to those of the flu or encephalitis. In at least 75 percent of cases, symptoms occur after an airway infection or a fever. Viruses observed before the development of the condition include Epstein-Barr virus, varicella zoster virus, herpes zoster virus, influenza A virus subtypes, and adenovirus. Several days after symptoms first occur, patients become very tired. In cases that occur after an infection, KLS usually starts within three to five days for teenagers and fewer for children. In other cases, alcohol consumption, head injury, or international travel precede symptoms. Lifestyle habits, such as stress, alcohol abuse and lack of sleep and stress, have also been proposed as possible triggers. First episodes of KLS are preceded by a clear event in about 90 percent of cases. Recurrences generally do not have clear triggers; only about 15 percent have a precipitating event.