The fast-spreading delta variant of the novel coronavirus has been gaining traction worldwide since it was first detected in India, and now researchers may have an idea of why: Its viral load is 1,000 times higher than the original strain of the virus.

The delta variant is a mutated version of the novel coronavirus that spreads more easily than other strains. It now accounts for an estimated 83% of COVID-19 cases in the United States as it continues to surge largely among unvaccinated populations, officials said.

Viruses constantly mutate, and most changes aren’t concerning. But there is a worry that some variants might evolve enough to be more contagious, cause more severe illness or evade the protection that vaccines provide.