Toban Nichols still has breathing issues and episodes of "brain fog" eight months after coming down with the virus. Among the millions who have gotten sick, many have suffered from long-term and sometimes debilitating heart, lung and nervous-system impairments. Experts still don’t know why.

<p>Toban Nichols still has breathing issues and episodes of "brain fog" eight months after coming down with the virus. Among the millions who have gotten sick, many have suffered from long-term and sometimes debilitating heart, lung and nervous-system impairments. Experts still don&rsquo;t know why.</p>