Treating one disease caused another.

Hepatitis C, a viral infection that can lead to liver failure, is surprisingly common in Egypt. Up to 20 percent of people there test positive for the chronic illness.

Now, a study of almost 8,500 Egyptians shows that reused needles spread the disease. Since the 1920s, health authorities employed needles in mass campaigns against schistosomiasis, a common illness caused by a blood-borne parasite. The needles were disinfected with procedures considered sufficient at that time. Like hepatitis C, schistosomiasis is a chronic disease with serious complications in some people.

"Egypt's extensive and dedicated nationwide control program for schistosomiasis was the cause of the current high prevalence of hepatitis C in the country," says G. Thomas Strickland of the University of Maryland School of Medicine in Baltimore.

The people who received the most injections were most likely to develop hepatitis C, Strickland and his team report in the March 11 LANCET. The gradual introduction of oral medications for schistosomiasis in the 1980s coincided with a drop in hepatitis C prevalence in younger people. The injection campaign may account for the world's largest transmission of blood-borne pathogens from medical intervention, Strickland says.