Seroconversion is not the same as infection. Instead, it refers to the changes in the body that allow a disease to be detected by a diagnostic test.
Many blood tests for sexually transmitted diseases - including some syphilis tests, herpes tests, and HIV tests - work by looking for the body's reaction to an infection rather than the infection itself. For example, tests such as ELISA tests and western blots are often used to look for the presence of antibodies against a particular infection.
It takes time for the human body to mount an immune response after an infection has taken place. First the body has to recognize that it is in danger, then it has to respond to that danger. One of the elements of that response is that the body makes different kinds of antibodies (IgG, IgM, IgA, etc.) that are specific to that illness. If a person has been infected with that disease before, an antibody response may happen quite quickly. If they haven't been, it can take substantially more time.
Seroconversion literally means "change of the blood," and, in the context of disease testing, the "change" that is being looked for is one that will show up on a diagnostic test. Usually this means that enough antibodies have been produced after an infection for doctors to detect them, but not always - some blood tests look for specific proteins produced by a pathogen, instead.
