This is the case with sexually transmitted diseases (or infections).
Sexually transmitted diseases are those that are passed from one person to another via sexual contact. These diseases include herpes, chlamydia, AIDS and many others.
The reason that these diseases are not detected is because sometimes the pathogen, the agent that causes the disease, has a long incubation period. Some STDs don't show symptoms for weeks, and some don't show symptoms for months even. Therefore, it is difficult to detect these diseases in individuals sometimes.