What is ivermectin, and how did people get the idea it can treat covid?

correction

In an earlier version of this story, David Boulware incorrectly cited an amount and concentration of animal ivermectin available online and calculated that it would be 100 times the normal human dosage, leading to toxicity. Ivermectin, at the amounts and concentration typically available online, would be more than seven times the normal dosage for an adult human, which could still lead to toxicity. This story has been corrected.