Breastfed babies rarely get ill so it's not very likely that baby will get a fever while 100% breastfed. However, your baby might have an older sibling at preschool who brings home a bad cold, which is passed on. If this happens and a fever develops, it's a healthy and normal response and shows that the immune system is working well. If your baby develops a fever, just keep him warm, and continue to feed regularly as normal. Don't give any pharmaceuticals to lower the fever, as this can prolong the problem. The fever needs to do its work - to fight the infection.
What has caused the fever? It could be a virus or bacteria. There may be an ear infection, tonsillitis, bronchitis, measles, chickenpox or any other common respiratory infection.