Stress fractures in healthy individuals are usually the result of sudden increases in physical activity over a period of a few to several weeks. Simply put, they are almost always a form of training error of doing too much too soon. They usually occur in weight bearing joints in which the over activity causes microscopic fractures that do not have enough time to heel (activity>rest) and results in macroscopic bone failure (fracture). This in turn leads to eventual pain during activities and sometimes even with everyday activities depending on extent. Early diagnosis can be made by MRI, and usually after 4 weeks with x-rays.
Treatment is obvious rest for the time period until palpation, pain and scans clear usually around 6 weeks. Some may require a boot or brace depending on extent. Proper nutrition and supplementation (think leafy greens, healthy fats, Vitamin D, Fish oil), adequate rest and non-impact cardiovascular exercise can help expedite recovery.