Kerri Phithibeault gives Danny Garcia a mpox vaccination in Orlando, Florida, in August.
CNN  — 

The US public health emergency declaration for mpox, formerly known as monkeypox, ends Tuesday.

The outbreak, which once seemed to be spiraling out of control, has quietly wound down. The virus isn’t completely gone, but for more than a month, the average number of daily new cases reported to the US Centers for Disease Control and Prevention has hovered in the single digits, plummeting from an August peak of about 450 cases a day.

Still, the US led the world in cases during the 2022-23 outbreak. More than 30,000 people in the US have been diagnosed with mpox, including 23 who died.

Cases are also down across Europe, the Western Pacific and Asia but still rising in some South American countries, according to the latest data from the World Health Organization.

It wasn’t always a given that we’d get here. When mpox went global in 2022, doctors had too few doses of a new and unproven vaccine, an untested treatment, a dearth of diagnostic testing and a difficult line to walk in their messaging, which needed to be geared to an at-risk population that has been stigmatized and ignored in public health crises before.

Experts say the outbreak has taught the world a lot about this infection, which had only occasionally been seen outside Africa.

But even with so much learned, there are lingering mysteries too – like where this virus comes from and why it suddenly began to spread from the Central and West African countries where it’s usually found to more than 100 other nations.

How long has it been spreading?

Before May 2022, when clusters of people with unusual rashes began appearing in clinics in the UK and Europe, the country reporting the most cases of mpox was the Democratic Republic of Congo, or DRC.

There, cases have been steadily building since the 1970s, according to a study in the CDC’s Morbidity and Mortality Weekly Report.

In the DRC, people in rural villages depend on wild animals for meat. Many mpox infections there are thought to be the result of contact with an animal to which the virus has adapted; this animal host is not known but is assumed to be a rodent.

For years, experts who studied African outbreaks observed a phenomenon known as stuttering chains of transmission: “infections that managed to transmit themselves or be transmitted from person to person to a limited degree, a certain number of links in that chain of transmission, and then suddenly just aren’t able to sustain themselves in humans,” said Stephen Morse, an epidemiologist at Columbia University’s Mailman School of Public Health.

Informally, scientists kept track, and Morse says that for years, the record for links in a mpox chain was about four.

“Traditionally, it always burned itself out,” he said.

Then the chains started getting longer.

In 2017, Nigeria – which hadn’t had a confirmed case of mpox in more than four decades – suddenly saw a resurgence of the virus, with more than 200 cases reported that year.

“People have speculated maybe it was a change in the virus that allowed it to be made better-adapted to humans,” Morse said.