Scientists say it is possible that if someone suffers from swine flu and seasonal flu at the same time, a drug-resistant pandemic flu virus could be created.
The new strain could arise if a cell is infected by both flu viruses, one of which is drug-resistant.
A New Zealand study, published in an American journal, tracked 11 people who have the co-infection, which researchers say is rare.
Lead researcher Matthew Peacey says that with influenza, when two viruses affect the same cell there's a possibility that if one is drug- resistant, the resistance could be passed on to the one that isn't.
However, Dr Peacey says this was not found to have occurred in this study.
