hundari, to my understanding rash is not khatira at all (rash contains almost no pus or little pus). pimple is somewhat close term but not exactly the khatira (a khatira contains more pus than a simple pimple does). Khatira (more than one khatiro) generally appear in groups and are called boils i guess. Better google for the terms you are looking for: http://www.medicinenet.com/boils/article.htm
http://www.aboutkidshealth.ca/HealthAZ/Boils-Abscesses.aspx?articleID=8383&categoryID=