When people say “contagious,” this means that your specific kind of contact easily transmits an ailment. Obviously, contagious diseases are infectious, spread by either physical contact from person to person, or by exposure to a realtor for instance a mosquito or tick carrying the herpes simplex virus or infectious material.
Suffice it to suggest, ADHD is not contagious — being around people who have ADHD do not put you at risk from then contracting ADHD. Even though a mosquito bites someone with ADHD and bites you next, you will find still zero chance you will have ADHD because the mosquito bite. Instead, ADHD is hereditary and genetic and can be learned in your children. No amount of contact and mingling with ADHD children and adults are likely to get you to “catch” ADHD.
References
https://kidshealth.org/en/teens/contagious.html
http://www.afrivip.org/sites/default/files/contagious.pdf
http://pedsinreview.aappublications.org/content/31/2/56