Three years in, the pandemic mania has settled to a rumbling hum. We’re again to sweating on one another in nightclubs, spluttering out birthday candles, and sharing agency handshakes. Covid-19, whereas nonetheless very a lot alive, has for most individuals diminished to an on a regular basis menace, because of vaccines and coverings.

The identical can’t be stated of lengthy Covid, the mysterious, life-limiting ailment that lingers on after an preliminary Covid an infection. For the thousands and thousands besieged by it, their scenario has remained a lot the identical. “We still have no established tools to help treat patients,” says Linda Geng, codirector of the Post-Acute Covid-19 Syndrome Clinic at Stanford University. Estimates of how many individuals have lengthy Covid fluctuate, but it surely’s been put as excessive as round 65 million—about the identical because the inhabitants of France.

It’s solely now, over three years into the pandemic, {that a} consensus on what lengthy Covid is has began to solidify. And what it’s, it seems, is an entire bunch of issues. Rather than a single dysfunction, it’s extra possible a smorgasbord of ailments that fall beneath one massive umbrella. That means there possible gained’t be a one-size-fits-all therapy both.

What triggers lengthy Covid for you might not be what units it off for an additional. Perhaps your lengthy Covid is attributable to your immune system turning on you, attacking your physique—a phenomenon known as autoimmunity. So goes one idea. Or possibly it’s that splinters of the virus are hanging around your body lengthy after the preliminary an infection, maintaining your immune system’s engine revved as much as the purpose of exhaustion. Another idea is that SARS-CoV-2 causes long-lasting damage to sure organs or tissues. Maybe it’s {that a} Covid an infection reawakens latent viruses your physique has encountered earlier than, such because the Epstein-Barr virus, which causes mononucleosis.

All these theories have some proof to help them, and so they might not be mutually unique; for some folks, this stuff could possibly be occurring on the identical time. The concept that lengthy Covid has completely different causes may go a way towards explaining the sheer variety of signs, which quantity up to 200.

Working off this foundation, researchers are attempting to hit two birds with one stone: trialing remedies that would alleviate lengthy Covid whereas on the identical time lending weight to sure hypotheses—and starting to defog the mystifying situation. “The reality is that there’s such an urgency, we need to do these things in parallel,” says Geng. “It’s building the ship as we sail it—but we have to sail it because people need help.”

But the jumble of signs makes designing medical trials a lot trickier. Not each particular person experiences each symptom, and people might fluctuate in severity and period. Plus, there’s no consensus on how one can outline lengthy Covid, says Steven Deeks, a doctor and infectious illness specialist on the University of California, San Francisco. “There’s no magic biomarker, there’s no x-ray, there’s no test.” Because of that, it’s powerful to determine who to place right into a medical trial. Right now, lengthy Covid diagnoses work by exclusion: figuring out that signs can’t be defined away by one other trigger. Regardless, researchers are plowing forward.