Can Defend be applied directly on people who are infested with lice?

No. Defend is designed to kill adult and nymph-stage lice on clothing, bedding, furniture and other inanimate objects. It is also effective against bed bugs, fleas and ticks, including ticks which transmit Lyme Disease. It will repel mosquitos as well, but all of these applications are predicated on the use of the product on (primarily) fabrics and upholstery, rather than directly onto people.