In our little roundup of sexually transmitted infections, maybe you’ve noticed that many of them have no symptoms, and others mimic common illnesses like the flu or a urinary tract infection.

The take-away? There’s no way to know if you’re carrying an STI without getting tested. And there’s no way to prevent reinfection unless your partner gets tested, too.

Viral Infections

That said, here’s an overview of the most common viral sexually transmitted diseases. That’s right, they’re caused by viruses, and you know what that means. You may well be stuck with this bug and at risk of infecting others for the rest of your life.

Here’s the list of bad boys:

Herpes Simplex Virus (HSV)

How common is it?

Very. One in five women has genital herpes. There are two types: Type 1 is the oral virus that causes cold sores. Type 2 infects the genital area. But the oral virus can infect the genitals and vice versa.

How do you catch it?

Through genital contact or oral-genital contact. The affected area can be contagious even when the lesions have healed. Condoms can help reduce infection, but it may not cover all the affected areas.

What are the symptoms?

The first outbreak is the worst and usually occurs within two weeks of infection. It may be accompanied by flu-like symptoms as well as pain and burning in the area of the lesions. But many people hardly notice the infection. Several less severe episodes may occur in the first year, then with lessening frequency and severity after that.

As with other STIs, herpes compromises the immune system, making the person more susceptible to new infections.

Human Papillomavirus (HPV)

How common is it?

HPV is the most common viral STI in the U.S. today. There are many strains of HPV and about half of sexually active people have one or more of them.

How do you catch it?

Through genital contact during sex, even though the carrier may not have symptoms. It can also be passed through genital-oral contact. A person can carry—and pass on—the virus for years without knowing he or she has it. A person may also be infected with more than one strain of HPV. Latex condoms offer some protection, but the infected skin may not be covered by a condom. “To be most effective, they

[condoms] should be worn with every sex act, from start to finish,” according to the CDC.

What are the symptoms?

HPV is usually asymptomatic, and about 90 percent of infections spontaneously clear up after a year or two, but for those that don’t some strains of HPV cause genital warts and others cause cervical cancer.

Genital warts (considered a low-risk virus) may be small bumps of various shapes and sizes on the genital area that appear weeks or months after infection, even if the partner doesn’t know he or she is a carrier. They may go away, become smaller, or grow, but they don’t become cancerous.

Cervical cancer (considered high-risk) has no symptoms until it is advanced. Abnormal cells are usually found in a Pap test; that’s why regular screening is important to determine whether the virus is high-risk.

Vaccines are available against several strains of HPV

HIV/AIDS

How common is it?

One million people have HIV/AIDS in the U.S., according to the National Institute of Allergy and Infectious Diseases. About 20 percent don’t know they’re infected. About 50,000 new cases are diagnosed every year.

How do you catch it?

Through contact with vaginal fluid, sperm, blood, and breast milk of an infected person. The virus is most contagious shortly after a person is infected, but it takes from 3 to 6 months for an accurate diagnosis of infection. Before that, tests results can yield false negatives.

People who have another STI are up to five times more likely to contract HIV/AIDS. People who have HIV/AIDS plus another STI spread the disease more effectively. For example, levels of the HIV virus are 10 times higher in men with gonorrhea. This is why it’s critically important to be tested and treated for other STIs. It’s also critically important to use latex condoms with a sex partner until you’ve both been tested and are sure enough time has passed for the test to be accurate.

What are the symptoms?

There may be no symptoms at first or the infected person may experience and intense flulike episode within a month of infection. The “flu” goes away; the system begins to produce antibodies to fight the infection, and the disease appears dormant.

Eventually, however, after several years, the compromised immune system collapses, and full-blown AIDS develops. Other infections take hold—pneumonia, diarrhea, and the telltale lesions of Kaposi’s sarcoma.

Medical science has progressed to the point that, with daily medication and close monitoring, a person can live with HIV for many years without the disease progressing to AIDS.

So, there you have it. But before you join a convent, remember how nice sex is, and especially with a special someone. Chances are, you both are infection-free. But why take chances when the stakes are so high and prevention is relatively easy?