When someone's infected, they've been exposed to an organism that causes a disease. It's too late for an infected person to get a flu vaccine — they already have the flu.
In some cases, bodies of water can be described as infected, or contaminated with bacteria that can make people sick. Infected food or water supplies can make entire cities fall ill. And a classroom full of infected toddlers means that their teachers and families will probably catch their colds. Infected, from the verb infect, is rooted in the Latin inficere, "to spoil or stain."