China may see new coronavirus cases due to existing patients testing positive again.

South Korea is seeing new cases as a result of foreign nationals entering the country for treatment.

The potential decline in the usage of masks also presents a real threat of the re-emergence of a virus outbreak in the near-term.

China and South Korea are said to have passed the peak of the coronavirus outbreak. The risk of a second wave of the virus still remains, and precautionary measures cannot be softened in the near-term to prevent it from emerging.

There are three key factors that could trigger the coronavirus outbreak to intensify once again.

#1: Recovered coronavirus patients are testing positive

While small, some recovered coronavirus have started to test positive after initially testing negative.

A report citing direct data from quarantine facilities in Wuhan shows that 5% to 10% of patients that recovered from coronavirus and were discharged from the hospital tested positive again down the line.

A Wuhan doctor who tested positive after a second test reportedly told NPR that the authorities are not counting asymptomatic cases, which could lower the actual number of coronavirus cases in the nation.

The problem with discharged patients testing positive for coronavirus is that in most mild cases, symptoms can take days to even weeks to reappear. When infected individuals begin to interact with other people, it opens up the possibility of more infections.

Wuhan, the epicenter of the coronavirus, recently eased quarantine and has allowed individuals to enter the city. As China eases quarantine measures in the near-term, it is critical to quickly detect individuals that may be infected by the virus after recovering from it.

#2: Foreign nationals traveling to Asia

Within a span of several weeks, the new epicenter of the coronavirus has shifted from Asia to Europe and the U.S.

Based on data from worldometer, the U.S. currently has 104,256 confirmed coronavirus cases and Italy has 86,498 cases, both surpassing China on paper.

In the past several days, South Korea has stared to see an increased number of coronavirus cases directly as a result of foreign nationals traveling to South Korea testing positive within the country.

Many individuals in Europe and the U.S. have been traveling to South Korea for its efficient coronavirus testing and medical centers.

The new inflow of foreign nationals, however, is beginning to overload existing medical centers and staff, leaving the possibility of a second wave of coronavirus open.

Residents in South Korea said that they fear when they see individuals carrying trolley bags outside, as most of the new cases in late March have come from foreign nationals raveling to South Korea.

China has completely blocked off most foreign nationals from entering the country, and it remains to be seen whether countries recovering from the virus in the likes of South Korea, Singapore, and Vietnam are likely to take a similar approach.

#3: Declining mask users

Virologists and scientists in China, South Korea, and other recovering countries state that the biggest problem in the U.S. and Europe is the lack of people wearing masks.

George Gao, director-general of the Chinese Center for Disease Control and Prevention (CDC), told ScienceMag in an interview:

The big mistake in the U.S. and Europe, in my opinion, is that people aren’t wearing masks. This virus is transmitted by droplets and close contact. Droplets play a very important role—you’ve got to wear a mask, because when you speak, there are always droplets coming out of your mouth.

The motivation to wear masks in China and South Korea could also decline as the number of new cases drop, and it is crucial for individuals to continue wearing masks outside to prevent the spread of the virus.