(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.

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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 was sent 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.

As of Tuesday, the U.S. had identified 306 cases in 27 states and the District of Columbia. More than 4,700 cases have been found in more than 40 other countries outside the areas of Africa where the virus is endemic.

There have been no U.S. deaths and officials say the risk to the American public is low. But they are taking steps to assure people that medical measures are in place to deal with the growing problem.

One of the steps was to expand who is recommended to get vaccinated. Vaccines customarily are given to build immunity in people before they are ever infected. But if given within days or even a few weeks of first becoming infected, some vaccines can reduce severity of symptoms.

A two-dose vaccine, Jynneos, is approved for monkeypox in the U.S. The government has many more doses of an older smallpox vaccine — ACAM2000 — that they say could also be used, but that vaccine is considered to have a greater risk of side effects and is not recommended for people who have HIV. So it’s the Jynneos vaccine that officials have been trying to use as a primary weapon against the monkeypox outbreak.

So far, the government has deployed over 9,000 doses of vaccine. U.S. officials on Tuesday said said they are increasing the amount of Jynneos vaccine they are making available, allocating 56,000 doses immediately and about 240,000 more over the coming weeks. They promised more than 1 million more over the coming months.

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The Associated Press contributed to this report.