Fungus; OR!!!! Mold.
The moisture gets to the strawberries after a while and the bacteria will grow into little colonies until the mold forms.
Strawberries (and other foods the have sugars) have things called 'Mold Spores' all over them. The mold will come out of the mold spores eat the fruit. This mold is not at all harmful to the human...
Keep them in the fridge to keep them fresh. They will grow mold eventually though, you can't prevent it forever.
put them in a bag with a lot of air in it or leave them in the fridge for a long time