Definitions:
a call to someone to participate in a competitive situation or fight to decide who is superior in terms of ability or strength

a call to prove or justify something

the administration of an immunogenic or infectious agent to an animal or person, in order to study the resulting immune response or measure the efficacy of a vaccine

invite (someone) to engage in a contest

dispute the truth or validity of

administer an immunogenic or infectious agent to an animal or person, in order to study the resulting immune response or measure the efficacy of a vaccine