“Regards to your question, it was noticed by our previous collaborator as well (see linked paper), that the co-infection with other bacteria is common, while co-infection with multiple viruses is rare.
According to Dr. Brunstein, this may due to the fact that the first viral infection may trigger the innate immune system and that will prevent the secondary viral infections; on the other hand, if a patient infected with a virus first, he or she may not have adaptive immune response to help prevent a secondary bacterial infection. That maybe why there are more bacterial-viral co-infection than viral-viral co-infection."