Some problems with phage treatment for STIs are that it can not go after bacterial STIs, there are not any phages that can be used to make a phage combination, and the focus is on model bacteria instead of STI-causing bacteria. Phage engineering may be an option if researchers cannot identify naturally occurring phages that infect STI-causing bacteria. Phage-targeting phages, such as prophages, can be artificially modified to destroy their hosts.