The World Health Organization (WHO) declared Thursday that the deadly coronavirus that has killed 171 people in China and spread to nearly 20 countries is now a global health emergency.

The United Nations health agency’s emergency designation means the outbreak is an “extraordinary event” that requires global coordination.

Speaking to reporters in Geneva, WHO Director-General Tedros Adhanom Ghebreyesus noted the worrisome spread of the virus between people outside China.

The main reason for this declaration is not because of what is happening in China, but because of what is happening in other countries, he said. “Our greatest concern is the potential for this virus to spread to countries with weaker health systems which are ill-prepared to deal with it.”

“This declaration is not a vote of non-confidence in China. …On the contrary, WHO continues to have the confidence in China’s capacity to control the outbreak,” he said.

The development comes after U.S. public health officials confirmed the country’s sixth virus case — the first person-to-person transmission — in the state of Illinois. The Centers for Disease Control and Prevention (CDC) said the patient lives with a Chicago woman who was previously diagnosed with the virus after visiting Wuhan, China, the epicenter for the ongoing epidemic. As of Thursday, two cases have been confirmed in California and Illinois, one in Washington State, and another in Arizona.

“We understand this may be concerning, but based on what we know now, our assessment remains that the immediate risk to the American public is low,” CDC Director Robert Redfield said in a statement.

“We anticipated this,” said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University. “The kind of contact that you have in a household is very close and very prolonged. That’s the kind of circumstance where we would anticipate a virus such as this could be transmitted.”

The ongoing outbreak has prompted some Republicans in Washington, DC, to call on the Trump administration to crack down on travel from China. Of the WHO’s declaration, Sen. Tom Cotton tweeted, “This declaration is a week overdue, but a welcome development. China must open completely to WHO and international scientists. And it’s more urgent than ever to stop travel between China and US.”

This week, Cotton urged officials to implement a “target travel ban,” citing China’s previous lack of transparency in fighting illness outbreaks.

“Given the latest developments and the many unknowns about this virus, we ought to follow Benjamin Franklin’s maxim: an ounce of prevention is worth a pound of cure. America is blessed with world-leading researchers and laboratories on the cutting edge of medical science and epidemiology. Working in tandem with them, I’m confident our federal research agencies can develop a vaccine in record time,” he wrote in a letter to Secretary of State Mike Pompeo, Health and Human Services Secretary Alex Azar, and Acting Homeland Security Director Chad Wolf.

The Associated Press contributed to this report.