In the first three months of 2019, the number of smallpox cases worldwide rose more than in the same period last year, according to the World Health Organization.
These interim data indicate a "clear trend" and epidemics are recorded in all parts of the world.
In Africa, the most damaging growth was recorded – over 700%.
WHO has announced that the actual numbers may be significantly higher, as only one in 10 cases of the disease is reported globally.
Smallpox can lead to serious health complications, including lung and brain diseases.
Ukraine, Madagascar and India are the countries where smallpox is the most widespread – tens of thousands of cases are registered per million inhabitants.
In Madagascar alone, since September last year, at least 800 people have died.
The epidemic also hit Brazil, Pakistan and Yemen, "causing numerous deaths – especially children," while there has been an increase in cases in countries where vaccination is high, including the United States and Thailand.
In 170 countries, 112,163 cases of smallpox were recorded, while in the same period in 2018, in 163 countries, there were 28,124 cases.
According to data from the Serbian Public Health Institute "Dr Milan Jovanovic Batut" in Serbia, four cases of smallpox have been reported in Serbia since the beginning of 2019 – one in Aleksinac and Bujanovac and two in Belgrade.
Last year 5,076 people were registered.
Most patients – 94 percent – were not vaccinated, vaccinated incomplete or had unknown vaccine status.
The United Nations said the disease was "completely preventable" by a real vaccine, but global coverage of the first phase of immunization "halted" to 85%, and "the 95% level of the vaccine has not yet reached what is necessary for prevent an outbreak of epidemics. "
In a statement released by CNN, WHO chief Tedros Adan Gebrayesus and UNICEF chief Henrietta Fore said the world was "in the midst of a smallpox crisis" and that it was wrong to disseminate contagious and contradictory vaccine information.
Gebrejesus and Fore claim that it is "understandable in such a climate that parents are lost," but that "after all, there is no debate about the essential benefit of the vaccine."
"More than 20 million lives have been preserved thanks to smallpox vaccines only since 2000," they add.
Why did a sudden "global smallpox crisis" occur?
James Galager, a BBC correspondent in the field of health and science
It is one of the most dangerous viruses. However, nothing has changed in relation to him. It did not mutate and became more contagious or dangerous. Instead, the answers are completely human.
In the center there are two stories – poverty and misinformation. In poorer countries, fewer people are vaccinated and therefore the majority of the population is vulnerable.
This creates perfect terrain for major epidemics – for example, in the Democratic Republic of Congo, Kyrgyzstan and Madagascar.
But rich countries with high vaccination rates are also facing the crisis. This is due to the fact that groups of people choose not to vaccinate children because they believe in anti-virus messages on social networks.
It should be noted that these are relative numbers, and that WHO has announced that they are actually much larger. Smallpox is far from harmless, and every year about 100,000 people, mostly children, are affected by the disease.
In response to recent smallpox epidemics, immunization has become mandatory in some countries.
Vaccination is also mandatory in Serbia and parents who refuse can be fined. According to the Law on the Protection of the Population of Infectious Diseases, penalties may reach 150,000 dinars.
Last year, 119 parents who did not vaccinate their children were found guilty before the Juvenile Court in Belgrade, of which 36 parents were fined in the amount of 5,000 to 50,000 dinars, Tanjug said recently.
Education Minister Mladen Šarčević has recently announced the possibility of introducing tougher penalties for parents enrolling unvaccinated children at school, which has given rise to much controversy.
Italy has recently banned children under six from going to daycare centers if they have not been vaccinated, while parents who have dropped out of school and who have not been vaccinated have to pay fines.
In New York, penalties for non-vaccination have recently been imposed.