Why Might an Individual with an STD Need to Be

Why might an individual with an STD need to be checked for other STDs, even though he/she has no symptoms of any others?

A) Several STD-causing pathogens are known to 'piggy back' on each other, being carried into the genital tract together.
B) A diagnosis of an STD implies that the individual engaged in unprotected sexual intercourse. Several STDs are largely asymptomatic, so it is wise to test for a variety of typical STDs when the patient admits to or shows signs of engaging in such risky behavior.
C) They don't need to be tested-all STDs 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 STDs.