In South Africa, a small group of volunteers is waging an online battle against Covid-19 and vaccine misinformation. Much of it comes from abroad, Jack Goodman reports. When Sarah Downs tweeted about how her grandmother had died from Covid-19, there was one response she did not expect. "I had just posted a message on Twitter basically saying my amazing grandma passed away and she meant a lot to me," she says. "So then I had someone asking me, 'Well, was there an autopsy done? What were the autopsy results? You don't know that she passed away from Covid. She could have passed away from something else.'" She had run directly into a Covid-19 denier. They're a relatively small but vocal group who have a range of beliefs - but in general, many think coronavirus is a "hoax" or "not real", and are against public health measures and lockdown rules. Right now, one of their main preoccupations is campaigning against vaccines. But Sarah - studying molecular biology and infectious diseases - was the wrong person to troll.