A new study has revealed that viruses and bacteria may carry memories of infections they've created in the past, and can use these memories to their advantage.

By exploring transmission patterns of infections, researchers found that pathogens that remember the sex of hosts they had infected can make their current host sicker. In doing so, they improve their own transmission.

This new research, resulting from an international collaboration among Royal Holloway, University of London (UK), Center National de la Recherche Scientifique (France) and The University of Western Ontario (Canada), indicates that complex patterns of infection virulence in measles, chickenpox and polio—which have previously defied medical explanation—may be clarified by considering the effect of natural selection acting on pathogens that can remember their past.

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