A new study at the University of Cambridge has found that certain viruses that affect plants can actually help the plants attract more bees for pollination. In turn, the plant yields more seeds, making the virus helpful for evolutionary growth and the continuation of the plant’s survival.
Head of Cambridge University’s virology and molecular plant pathology group, John Carr said:
“We were surprised that bees liked the smell of plants infected with the virus – it made no sense. You’d think pollinators would prefer a healthy plant. However, modelling suggested that if pollinators were biased towards diseased plants in the wild, this could short-circuit natural selection for disease resistance.
The virus is rewarding disease-susceptible plants and at the same time producing new hosts it can infect to prevent itself from going extinct. An example, perhaps, of what’s known as symbiotic mutualism.”
Researchers are now discussing how this could have real world implications, helping crops grow faster and produce seeds more regularly. Carr says that to his knowledge, this is the first time there has been evidence that an infection or virus can make a plant more attractive for pollination.
According to the study, the cucumber mosaic virus works by infecting a plant just like any other virus. Making the plant its host, the plant begins to wither and decay, making it look half-dead. However, the virus also expresses itself by making the plant smell appealing to bees. 
Dr. Carr also had this to say about the experiment, which took 8 years of study:
“We would expect the plants susceptible to disease to suffer, but in making them more attractive to pollinators the virus gives these plants an advantage. Our results suggest that the picture of a plant-pathogen arms race is more complex than previously thought, and in some cases we should think of viruses in a more positive way.”
 The Guardian
 New Scientist