Advances in antiviral therapy have involved both development of new, effective drugs and modification of pre-existing drugs or regimens to increase effectiveness. New agents against influenza virus are the neuraminidase inhibitors zanamivir and oseltamivir, which target specific viral processes and have minimal side effects. New agents for herpesviruses (famciclovir, valaciclovir, valganciclovir) have greater oral bioavailability, allowing less frequent dosing, but mechanisms of action are similar to older agents (aciclovir, ganciclovir). Pleconaril has some activity against enteroviruses and is available for compassionate use in meningitis; it also shows some efficacy against rhinoviruses in ongoing trials, but is not available for routine clinical use. Hepatitis C treatment efficacy has improved dramatically with the introduction of combination interferon-ribavirin therapy, with sustained-response rates up to 60%.