Sexually transmitted diseases are very infectious and often most people have no idea they have them. Up to 40% of men and 80% of women who have an STD have no symptoms. They could be passing it onto others unknowingly.
This is where STD screening comes to the fore. As long as samples are collected properly the chance of identifying and STD is very high, even in those with no symptoms. This means the correct treatment can be administered, reducing long term complications such as infertility.