Most infectious diseases that were once deadly can now be prevented with vaccines or treated, except for some dangerous diseases like prion diseases (spongiform encephalopathy).
Infectious diseases account for 3 of the top 10 causes of death according to the World Health Organization (WHO), resulting in millions of deaths each year globally. According to Amesh Adalja, an infectious disease physician at the Johns Hopkins Center for Health Security, almost every infectious disease that once had a 100% mortality rate can now be prevented with vaccines or treated thanks to modern medicine. However, there are still deadly infectious diseases that humanity has yet to solve. Some of these diseases are always or nearly always fatal, while others have a very high mortality rate.
For example, amoebic meningoencephalitis—or “brain-eating” amoeba infection—is a rare infectious disease that is almost always fatal. The disease spreads to the brain through the nose, often occurring after the patient immerses themselves in contaminated water. In some rare cases, patients have been successfully treated, but scientists are still searching for better solutions.
A type of prion disease known as spongiform encephalopathy. (Photo: UCSF).
Some other rare diseases remain a mystery, such as prion diseases. These diseases are caused by misfolded proteins in the brain—known as prions. They cause other proteins to misfold in a chain reaction, ultimately leading to brain damage and death.
The majority of prion disease cases are not considered infectious. They arise from inherited or spontaneous gene mutations. However, in some rare cases, individuals can become infected after consuming prion-contaminated meat or through exposure during medical procedures; for example, variant Creutzfeldt-Jakob disease (vCJD) can be contracted after eating meat from cattle affected by “mad cow disease”, or Kuru, which occurs among the Fore people in Papua New Guinea.
“Prions have been studied for decades, but I think people are still trying to figure out the root causes,” said Rodney E. Rohde, an expert in infectious diseases at Texas State University. Although extremely rare, prion diseases share one common trait: once infected, there is no treatment, and death usually occurs within weeks after symptoms appear.
What makes these types of diseases so deadly? The evolutionary history of the disease is one reason. With a disease that has infected human hosts for tens of thousands of years, the human body has strived to build barriers against it, increasing survival chances. However, if humans are an incidental host or a dead-end host—like in the case of rabies—the disease does not aim to keep humans alive, as they are not the primary host. In such cases, humans often have not developed the appropriate immune response to fight back without medical assistance.
For example, rabies triggers an immune response in humans, but this response is not fast enough to defeat the virus before it reaches the brain and kills the host. “Some pathogens are inherently more vicious and malicious. They overwhelm the immune system so that the body cannot adapt in time,” Rohde shared.
