How common are unexplained outbreaks of disease?

Amidst reports of an unexplained respiratory illness spreading among children in China, it's worth examining how frequently such mysterious outbreaks occur. Let's take a trip back to 1993, when a Navajo family in New Mexico found themselves in a terrifying situation. Their 19-year-old son, a marathon runner in peak physical condition, suddenly developed severe breathing problems. Despite immediate medical attention, he tragically passed away. The medical team was left baffled, unable to determine what could have caused the sudden demise of such a healthy young man. It was later revealed that his fiancée had succumbed to a similar respiratory illness just days prior.

Whenever news of a mysterious new outbreak surfaces, there's a plethora of potential culprits. The Earth is teeming with more viral particles than there are stars in the universe, and our bodies host ten times more bacterial cells than mammalian ones. In total, our planet is home to an estimated one trillion species of microorganisms. However, only a small fraction of these are known to cause disease in humans. The rest remain undiscovered, potentially capable of sparking the next unexplained outbreak.

In November 2023, China's National Health Commission reported a surge in respiratory diseases, particularly among children. Hospitals were inundated with sick children, many of whom were diagnosed with pneumonia. Despite the alarming situation, Chinese officials informed the World Health Organization (WHO) that they had not detected any new pathogens. They attributed the outbreak to a combination of regular winter illnesses such as RSV and the flu. However, neighboring countries like India remain skeptical about this explanation, prompting the WHO to continue monitoring the situation. Unraveling the mystery behind such outbreaks can be a complex process. In the case of the Navajo couple in 1993, it took two months to identify the culprit as hantavirus pulmonary syndrome, caused by a completely new strain of hantavirus, a group of viruses typically found in rodents. Delays in identifying the cause of outbreaks are surprisingly common, even in the modern era. Many outbreaks initially go unnoticed and unreported. Eventually, a medical expert might take a particular interest in an infection, leading to an increase in reports. However, in remote areas or places lacking technical facilities, many outbreaks simply don't get noticed. This was the case with Covid-19, which was officially identified a month after the WHO was alerted to a cluster of pneumonia cases in Wuhan, China, in December 2019. However, some research suggests that the virus may have begun spreading in humans as early as October 2019. Once an outbreak has been identified, the next challenge is to find the pathogen responsible. In the case of the 1993 outbreak, the virus was identified using the Polymerase Chain Reaction (PCR), a then cutting-edge technology. Today, PCR is a standard method for identifying pathogens, but it has its limitations. For instance, it requires a sequence from something closely related to the pathogen being sought, making it difficult to identify the cause of an outbreak without a clue. However, more sophisticated methods for identifying previously unknown pathogens are now emerging. One such method is next-generation sequencing, which can help scientists discover microorganisms that are completely new to science. Despite these advancements, the causes of some outbreaks remain elusive. For instance, in 2010, an unknown hemorrhagic illness began spreading in northern Uganda. Some samples tested positive for yellow fever, but others did not, leaving the cause of the outbreak uncertain. Therefore, by the time a mysterious outbreak makes global headlines, scientists are often already on the path to discovering its cause, provided they have the necessary resources.