I accidentally picked my nose after touching a public doorknob will i get sick?

Little or no risk: If someone with a cold, flu, or a gastrointestinal infection had very recently used the doorknob, there might be a slight risk but extremely unlikely. I'm wondering if you're a germophobe, i.e. abnormally fearful of contamination. If such fear dominates your life, consider counseling. It can be an early sign of serious mental health disability -- rent "The Aviator" to see where this can lead.