Last week, researchers at the Los Alamos National Laboratory released alarming news: At least one variant of the virus that causes Covid-19 had significantly mutated to become more contagious. If true, this would have major implications. A new variant could, for example, hamper efforts to develop a vaccine or mean that people who’ve already had Covid-19 might face a greater risk of getting it again. But critics said the data didn’t support such a big claim. Kristen V. Brown discusses what it really means that the virus is changing.