A virus is a submicroscopic infectious agent, composed of genetic material encased within a protein shell, that can only replicate inside the living cells of an organism. Viruses are not considered living organisms because they lack cellular structures and metabolic processes, requiring a host cell’s machinery to create new copies of themselves. While countless viruses exist, a select few have gained notoriety due to their significant impact on human history and health, often through severe disease or rapid spread.
Viruses Known for High Fatality Rates
Some viruses become widely known for their extreme deadliness, often measured by their Case Fatality Rate (CFR). This metric indicates the proportion of individuals diagnosed with a specific disease who ultimately die from it, reflecting the illness’s severity. Viruses like Ebola, Marburg, and Rabies exemplify this through the severe damage they inflict on the human body.
Ebola virus disease (EVD) has an average CFR of around 50%, though this has varied from 25% to 90% in past outbreaks. Initial symptoms include sudden fever, fatigue, muscle pain, and headache, progressing to vomiting, diarrhea, abdominal pain, and impaired kidney and liver function. In fatal cases, massive internal and external hemorrhaging can occur, contributing to high lethality. Marburg virus disease (MVD), a close relative of Ebola, also causes a severe hemorrhagic fever with a CFR reaching up to 88-90%. Its abrupt onset with high fever and severe malaise can quickly escalate to multi-organ failure and shock, often including bleeding from various orifices.
Rabies, caused by a lyssavirus, stands out with an almost 100% fatality rate once clinical symptoms appear. The virus travels along nerve pathways to the brain, causing inflammation. Symptoms initially resemble the flu but quickly progress to severe neurological manifestations like hydrophobia, hyperactivity, and an inability to swallow, or a paralytic form leading to gradual paralysis. Death typically results from cardiorespiratory arrest within days of symptom onset.
Viruses with High Transmission Rates
Other viruses achieve widespread recognition not primarily for their lethality, but for their ability to spread rapidly and extensively through populations. This characteristic is often described by the basic reproduction number (R0), which represents the average number of new infections generated by one infected individual in a completely susceptible population. A virus with an R0 greater than one indicates it will likely spread through a community.
Measles is considered one of the most infectious human pathogens, with an R0 often cited between 12 and 18. Its airborne transmission allows viral particles to remain suspended in the air for extended periods, facilitating widespread infection among susceptible individuals, even without direct contact. The 1918 Spanish Flu, caused by an H1N1 influenza A virus, also demonstrated high transmissibility, infecting an estimated one-third of the global population. While its R0 was estimated around 2 to 3, its rapid global spread was exacerbated by airborne respiratory transmission and crowded conditions during World War I.
More recently, SARS-CoV-2, the virus responsible for COVID-19, showed a high capacity for spread through respiratory droplets and aerosols. Early R0 estimates for SARS-CoV-2 ranged from approximately 2.7 to 4.2, indicating its potential for transmission. The virus’s ability to undergo frequent mutations, giving rise to new variants with altered transmissibility and immune evasion properties, has further contributed to its global circulation and pandemic impact.
Viruses Causing Chronic Infections
A different form of notoriety arises from viruses that establish long-term or lifelong infections within the human body. This persistence means the virus can remain dormant in host cells without actively replicating, thereby evading immediate immune detection. The viral genetic material can either exist as a separate entity within the cell or integrate into the host cell’s DNA.
Human Immunodeficiency Virus (HIV) is an example of a virus that establishes a chronic infection, gradually undermining the immune system over many years. As a retrovirus, HIV integrates its genetic code into the DNA of host cells, particularly CD4+ T cells. Over time, this viral presence leads to the destruction of these cells, weakening the body’s ability to fight off other infections, eventually progressing to Acquired Immunodeficiency Syndrome (AIDS).
Hepatitis C virus (HCV) also causes a chronic infection, primarily targeting the liver. Many individuals can live for years or even decades without noticeable symptoms while the virus silently causes inflammation and damage to liver tissues. Without treatment, this persistent inflammation can lead to severe complications such as cirrhosis and liver cancer, potentially culminating in liver failure.
Eradicated or Controlled Viruses
While many viruses continue to pose health challenges, some once-notorious viral diseases have been successfully conquered or brought under control through global scientific and public health efforts. This represents a triumph of medical science and coordinated action.
Smallpox, caused by the Variola virus, was a historically devastating disease that killed millions and left survivors disfigured or blind. Through a global vaccination campaign spearheaded by the World Health Organization (WHO), Smallpox was officially declared eradicated in May 1980. This achievement involved widespread immunization and a rigorous surveillance-containment strategy to isolate cases and vaccinate contacts. Smallpox remains the only human infectious disease to have been completely eliminated from the world.
Polio, a viral disease that can cause irreversible paralysis, is another success story on the brink of eradication. The Global Polio Eradication Initiative (GPEI), launched in 1988, has reduced polio cases by over 99% worldwide. Extensive vaccination campaigns have eliminated two of the three wild poliovirus types. Efforts continue to eliminate the last remaining type of wild poliovirus, now confined to only a few countries.