Why Might a Person with an STI Need to Be

Why might a person with an STI need to be checked for other STIs, even though he/she has no symptoms of any others?

A) Several STI-causing pathogens are known to coexist with each other, being carried into the genital tract together.
B) A diagnosis of an STI implies that the person had unprotected sex. Several STIs are largely asymptomatic, so it is wise to test for a variety of typical STIs when the patient admits to or shows signs of engaging in such risky behavior.
C) They don't need to be tested. All STIs are highly obviously symptomatic, so there is no need to test for other diseases than the one currently manifesting itself in the patient.
D) It's a public health issue. It's best to know who is positive for what in the population of sexually-active individuals. This lets healthcare professionals act to best safeguard the population as a whole from the range of STIs.
E) They don't need to be tested. When a person becomes infected with one STI, they automatically become immune to any others, so will not acquire any further infections.