Does the severity of a virus, like stomach viruses, dictate how contagious it will be? eg. no fever, only lasts a day = less severe = less contagious

Unrelated: There is no consistent relatinship between severity and transmissibility. It's often the opposite. Norovirus, the main cause of viral gastroenteritis ("stomach flu"), usually is mild and lasts a few days; and one of the most easily transmitted of all viruses. Ebola is iasily trasnmitted and usually fatal. HIV also is fatal if untreated, but hard to transmit -- only by sex, needle sharing, etc.