There is a very high prevalence of sexually transmitted infections without symptoms. People can and often are living with an STI for many years without knowing it. During that time, if they're not careful, they can transmit the infection to some or all of their sexual partners. Several of the more than 20 types of STDs can remain latent for months, years, or even decades.
Latent STDs and STIs highlight the importance of sexual health testing, since only one test can determine if there is an asymptomatic infection. Chlamydia, hepatitis C, HIV, HSV (herpes simplex virus), and syphilis may have periods of latency. While some STDs, such as HIV and syphilis, can stay in the body for a while before symptoms appear, they are generally known to be symptomatic. In most cases, an infected person will show the tell-tale signs of the infection.
However, there are some STDs that are known to be asymptomatic, meaning you'll never know you have them until they've spread or caused other side effects. Many STDs don't cause any symptoms that you might notice. The only way to know for sure if you have an STD is to get tested. You can get an STD by having sex with someone who doesn't have symptoms.
Like you, that person may not even know that they have an STI. It's possible to get an STI again (after receiving treatment) if you have sex with someone who has an STD. And you may not even know that you have an STI, either because the symptoms are very subtle or because there are no symptoms at all. That's why regular testing for STDs is essential to your health, and for the health of others it's the only way to know for sure if you have an STD.
A latent sexually transmitted disease is a disease or infection that exists in the body without causing symptoms of STDs. Since most STI tests use antibodies (not symptoms) as a marker of disease status, having symptoms is not necessarily a reliable marker of infection. While the terms STDs and STIs are often used interchangeably, it's worth noting that many of these ailments are, in fact, infections that don't have any symptoms and can be cured with antibiotics. Each STD has a different incubation period, meaning they vary in the amount of time it takes for symptoms to appear.