Pharyngitis may be caused caused by a viral infection (90% of cases) or by a bacterium (10%). If caused by the group A streptococcus, it is called "strep throat". The chronic form can be caused by a continuing infection of the sinuses, lungs or mouth, or by constant irritation from smoking, breathing heavily polluted air, food allergies or consuming too much alcohol, or by swallowing substances that scald, corrode or scratch the throat. Pharyngitis is a common illness. Even strep throat is usually a self-limiting disease. Controlled trials show that clinical recovery is similar in cases with and without the use of antibiotics. The "streptophobia" associated with the risk of rheumatic fever and post-streptococcal glomerulonephritis is probably unwarranted today, although proper care is still important. Epidemiological surveys demonstrate a correlation between vitamin C deficiency and the development of post-streptococcal consequences. Rheumatic fever is virtually nonexistent in the tropics where vitamin C intake is higher; 18% of children in high risk groups have subnormal serum vitamin C levels.