Struvite stones can be one of the most troublesome for patient and physician.  These are fast growing stones that grow to fill up the naturally occurring cavities in the kidney to take on a "staghorn" appearance.  Staghorns most commonly, but not always, occur with urinary tract infections which lead to the precipitation of magnesium amonium phosphate crystals; hence the name "infectious stones".  Non-infectious stones such as uric acid and cystine can also form large cavity filling staghorns as shown in the picture below. 

These stones are difficult to remove and often require extensive surgery.  The infection often gets deep into the stone where antibiotics cannot penetrate.  Therefore, many people will have recurrent infections.  Preventing recurrence involves not only full removal of the stone and infected fragments, but also effective sterilization of the urine and kidney with antibiotics.  Unfortunately, the infection is often difficult to eradicate and stones often recur.  Certain bacteria which cause an increase in urine pH, are associated with these stones.  A common cause is Proteus bacteria.  These stones can cause significant damage to the involved kidney.  Sometimes kidney removal prior to transplantion is necessary.