Here’s the thing about that antique shaving brush you bought on eBay: it might have anthrax on it. Poorly disinfected animal hair shaving brushes caused a mini-epidemic of head and face anthrax during World War I, according to a historical review published by scientists at the Centers for Disease Control and Prevention. Modern brushes or well-used antiques made after 1930 are probably safe, the authors of the study write.

The paper reviews outbreaks of anthrax infections of the skin during the early decades of the 20th century. Anthrax spores can get into the skin through a cut — like, say, if you nicked yourself shaving. There, the spores can cause swelling, itchy bumps or blisters, and also painless ulcers with a black center. Anthrax skin infections are the least deadly kind of anthrax infection, especially with treatment. But it can still kill one in five people who are infected if it’s left untreated.

During World War I, anthrax cases started cropping up in the US and UK. At first, officials with the British armed forces thought that the anthrax infections of soldiers’ heads and necks were because of “diabolical tactics of the enemy.” But contemporary researchers were eventually able to track the outbreak back to the shaving brushes provided to the soldiers. (Chemical weapons employed during WWI meant soldiers had to wear gas masks — and gas masks were thought to fit better on clean-shaven faces.)

The contaminated brushes were also, for the most part, counterfeit. When World War I cut off the regular supply of badger hair from Russia, the market opened up for knockoff badger hair — which was really horsehair from Russia, China, and Japan. Apparently, horses and other herbivores are more at risk for anthrax infections than badgers and pigs, which are omnivores. And while the real bundles of badger hair had been disinfected before they reached the US, these knockoff horsehair bundles weren’t.

Once public health officials figured out that the brushes were to blame, the surgeon general and the New York City Board of Health came up with new recommendations for disinfecting and labeling sterilized brushes. So, the risks from using a brush made in the US after 1930 are really, really low, the authors of the study write.

But let’s say you really want to shave someone with that pre-1930 brush and you don’t want to infect them with anthrax. You might be wondering if it’s possible to sterilize the brush yourself. Unfortunately, the study authors don’t recommend it — the best decontamination measures for anthrax (steam, pressure, formaldehyde) are dangerous themselves. It might be best to just keep that antique brush off your face.