variant –n. something that is different in some way from others of the same kind

mutate –v. to cause a gene to change and create a new, different quality in a plant or animal

immunity –n. the power to keep yourself from being affected by a disease

frequently –adv. happening often

evolve –v. to change slowly into a different, sometimes, more complex form with different qualities

circulate –v. to spread from one person or place to another

mild –adj. not strong or severe

let their guard down –idiom to stop being careful or concerned about possible risk