Is infected means they are currently infected. They currently have some sort of pathogen or disease in their system.
Has been infected may be a past tense of the above. Some time in the past, they had some sort of pathogen or disease in their system.
Both of the above refer to the state. ( adj infected child )
Has been infected may also be a verb form referring to the process of being infected rather than the state of infection.
There’s very little chance the child has been infected.
This may refer to the state or process either one. The context may give a clue. Either way, the end result is the same. You can’t have one without the other.
As a verb form, if they have been infected, they may or may not still be infected. Has been infected does not imply either one.
As a state of infection, has been infected strongly implies they are no longer infected.