It is a variant of the coronavirus, which has been discovered in more than 80 countries after it was first identified in India. It was named by the World Health Organization. Notable variants are named after letters in the Greek alphabet.
Viruses are constantly changing and many of these changes are not concerning. There is concern that some viruses might become more contagious or cause severe illness, or even evade vaccines.
Experts believe the delta variant spreads faster because it has more mutations that allow it to latch onto cells. The variant is responsible for 90% of all new infections in the United Kingdom. It is responsible for 20% of all new infections in the United States, and officials believe it could be the dominant type.
Research has shown that vaccines are effective against variants including the delta variant.
Researchers in England compared the effectiveness of two-dose AstraZeneca vaccines and Pfizer BioNTech vaccines against the virus to the alpha variant first identified in the U.K.
Both doses of vaccines protected the individual, but less so for those who only received one.
Experts agree that it is crucial to get fully vaccinated. It's also why experts say it's so important to make vaccines available worldwide.