The incubation period from infection to the onset of symptoms is usually one to two weeks, but can be much shorter (days), or much longer (months to years). Lyme symptoms most often occur from May to September in the northern hemisphere, because the nymphal stage of the tick is responsible for most cases. Asymptomatic infection exists, but occurs in less than 7% of infected individuals in the United States. Asymptomatic infection may be much more common among those infected in Europe.