(NewsNation) — Syphilis cases are on the rise in the U.S. and shortages of a key drug are limiting access to treatment for those with the illness.

Cases of the illnesses have been on the rise since 2000, including sexually transmitted infections among adults, as have congenital syphilis cases, which occur when mothers pass the disease to their unborn children. Cases reached a 70-year high in 2021, according to data from the Centers for Disease Control and Prevention.

Syphilis is typically treated with penicillin G benzathine, also known as Bicillin L-A, an injectable medication. It’s the only treatment recommended for pregnant people with the disease.

Syphilis is caused by the bacteria Treponema pallidum and is primarily passed through contact with syphilitic sores. It can also be spread from a pregnant person to their unborn child.

The disease can resemble many other conditions and has multiple stages of infection. If left untreated, it can result in organ failure or death.

While any sexually active adult is at risk for the disease, certain communities, including gay and bisexual men, are considered at higher risk. When it comes to congenital syphilis, Black and Hispanic communities have faced increased rates of the disease.

Experts are unsure why syphilis cases have increased at higher rates than other sexually transmitted infections. Some theories include the disruption of STI prevention and treatment programs during the pandemic, along with the general stigma around STIs. Those theories, however, do not explain why syphilis rates is outpacing other infections.

The FDA said the shortage of bicillin is due to rising numbers of syphilis cases, as well as the drug’s use as an alternative to amoxicillin. Amoxicillin has been in short supply due to surges in cases of respiratory illness and severe strep throat.

Pfizer, which is the only company manufacturing penicillin G benzathine, said it would take about a year to increase production enough to end the shortage.

Experts fear the shortage could make it difficult for patients to get treated. The CDC is recommending providers give preference to pregnant patients and infants who have been infected or exposed, while suggesting an alternate drug regimen for others.

Doxycycline, the alternative treatment, is not recommended for pregnant patients and has a more complicated, twice-a-day dosing regimen that all patients may not follow.