(NewsNation) — Health officials are monitoring a developing outbreak of monkeypox, but this isn’t the first outbreak in the U.S.

The Centers for Disease Control and Prevention defines monkeypox as a “rare but potentially serious viral illness that typically begins with flu-like illness and swelling of the lymph nodes and progresses to a widespread rash on the face and body.” Monkeypox is similar to smallpox, with cases typically coming from West or Central Africa.

In 2003, 47 “confirmed and probable cases” were reported across six states, Illinois, Indiana, Ohio, Wisconsin, Kansas, and Missouri, all of which were linked to people who had had contact with pet prairie dogs, according to the CDC.

Investigators traced the outbreak to a shipment of animals that were imported from Ghana to Texas. The shipment contained around 800 small mammals, including six types of African rodents.

Then, an Illinois animal vendor housed some of those infected animals near prairie dogs, which ultimately led to the prairie dogs becoming infected. The dogs were purchased, then passed monkeypox on to their new owners and others with whom they came in contact.

So, how was it contained?

The CDC, several federal agencies, and public health departments did extensive laboratory testing, deployed smallpox vaccines, and immediately restricted the importation of African rodents.

During that time, monkeypox was never transmitted through person-to-person contact, but only from handling the infected animals.

In 2021, two monkeypox cases were reported in the U.S. In both instances, the patient had recently traveled to Nigeria. After contact tracing and waiting for potential symptoms to occur in people with possible contact, no other cases were identified.

Meanwhile, the 2022 outbreak has spread through person-to-person contact. Health officials said the illness is spreading through sexual contact. That intimate contact is believed to be how it passes rapidly from human to human.