The 1918 flu epidemic was probably the deadliest plague in human history, killing more than 50 million people worldwide. Now it appears that a small number of the deaths may have been caused not by the virus, but by a drug used to treat it: aspirin.

Dr. Karen M. Starko, author of one of the earliest papers connecting aspirin use with Reye’s syndrome, has published an article suggesting that overdoses of the relatively new “wonder drug” could have been deadly.

What raised Dr. Starko’s suspicions is that high doses of aspirin, amounts considered unsafe today, were commonly used to treat the illness, and the symptoms of aspirin overdose may have been difficult to distinguish from those of the flu, especially among those who died soon after they became ill.

Some doubts were raised even at the time. At least one contemporary pathologist working for the Public Health Service thought that the amount of lung damage seen during autopsies in early deaths was too little to attribute to viral pneumonia, and that the large amounts of bloody, watery liquid in the lungs must have had some other cause.