Mold grows on fruit by infecting the fruit with spores that develop filamentous roots that sink below the surface of the fruit. These roots steal nutrients from the fruit and are often not visible to the naked eye. The part of the mold that can be seen is the stalk above the roots and the mold's spores.
Molds prefer warm, humid conditions both indoors and outside though they can grow in cooler temperatures, such as those found in the refrigerator. Spores are released from mature mold and float through the air, landing on a new host and infecting it.
Soft foods and foods that contain high levels of moisture are most vulnerable to molds. These types of foods are easier for the roots of the mold to penetrate. These foods are particularly dangerous since the mold may spread farther into the food than the eye can see, creating a false sense of security for someone who chooses to eat the food after trimming off the mold.
Most food that has mold on it should be discarded. If the mold spot is small, it may be trimmed off firm fruits and vegetables. Mold can be prevented by covering foods and storing them in the refrigerator as soon as possible.