An analytic derivation of the dispersion relation for resistive instabilities in a low-shear tokamak configuration is presented. The resistive infernal mode model (Charlton et al 1989 Phys. Fluids B 1 798) is generalized to include plasma diamagnetism, subsonic equilibrium toroidal flow shear and viscosity. An estimate of the transition point between the fast S−3/13 infernal-like (S is the Lundquist number) and the slow S−3/5 tearing-like scaling is given. A novel S−3/8 scaling is found close to the ideal ion-diamagnetic magnetohydrodynamic stability boundary. New moderately fast scalings in S are also found when sheared toroidal E × B flow and viscosity are considered. An analytic treatment of the m = n = 1 quasi-interchange mode in presence of density gradients with flat temperature profiles has been made.