The condition known as long covid continues to frustrate its sufferers, baffle scientists and alarm people who are concerned about being infected by the coronavirus. The term, a widely used catchall phrase for persistent symptoms that can range from mild to debilitating and last for weeks, months or longer, is technically known as Post-Acute Sequelae of SARS-CoV-2 infection, or PASC. But scientists say much remains unknown about long covid, which is also referred to colloquially as “long-haul covid,” “long-term covid,” “post-covid conditions” and “post-covid syndrome,” among other names.