Pilonidal sinus is a hole or small canula like structure created by hair piercing in delicate skin of anal or surrounding anal region.Due to sweat and unhygienic conditions it may get infected developing symptoms.
In your case it is from birth and not infected till now.
Next Steps
Keep proper hygiene.Remove hair and avoid growth of unwanted hair in and around anal region. Wear lose and airy clothes.
Health Tips
Wash it after every washroom visit. Keep it dry and clean. This will surely prevent infection in that area.