In the late 1800’s, scientists were working to characterize the means through which microbes caused disease. While certain diseases like cholera, tuberculosis and anthrax could be explained by the presence of specific bacteria, other diseases, like rabies, could not. As such, these agents were considered to be a “biological chemical”; accordingly, they were named virus, from the Latin word referring to poison or other venomous compounds. At this time, scientist Martinus Beijerinck was studying the infectious agent that caused tobacco mosaic disease, a plant disease responsible for widespread crop damage. Through his studies, Beijerinck proposed that the virus that caused this disease must be much smaller than bacteria, as it cannot be visualized by light microscopy. Furthermore, since viruses are unable be cultured in nutrient media, they must only be able to replicate in their host organism. Wendell Stanley confirmed the theory that viruses were infectious particles 1935, when he visualized the tobacco mosaic virus using an electron microscope.
Today, virologists have shown that viruses are very simple infectious agents, comprising little more than a DNA or RNA genome surrounded a protective protein coat. Viruses rely entirely on a host organism for basic biological functions, including DNA replication, transcription, and basic metabolism. For example, viruses lack ribosomes, the intracellular machinery responsible for protein synthesis. Therefore, for viral RNA to be translated into protein, it must take over the translation machinery of its host organism. Early experiments characterizing the life cycle of viruses contributed to early discoveries in molecular biology, lending valuable insights to the processes of DNA replication, transcription, and translation.
Viruses represent a major public health concern because these infections impact the health of their host (Table 1). Some viral infections, like the common cold, are relatively harmless. The majority of cold symptoms (fatigue, fever and body aches) result from the body’s innate response to the infection, not the virus itself. Other viruses, like Human Papillomavirus or Hepatitis B, integrate their genes into the host genome where they can influence the behavior of the cell, transforming a normal cell into a cancer cell. Still other viruses, like smallpox or HIV, can be deadly.
Over the past few years, several viruses have captured the headlines because of widespread outbreaks in the United States and around the world.
- Influenza: Each year, scientists predict which strains of influenza will be most common in a particular flu season and formulate a vaccine to combat them. In some years, the predictions are very accurate, resulting in a highly-effective vaccine. In other years, the most common flu strains are not represented in the vaccine, reducing its effectiveness. While this means that the virus will infect more people, the vaccine still decreases the severity and the duration of the infection.
- Enterovirus D68: This virus causes severe respiratory distress, especially in individuals with asthma. Although researchers in California first identified the virus in 1962, infection by EV68 remained rare until 2014 when the virus caused a nationwide outbreak.
- Ebola: The Ebola virus was first identified in 1976 in today’s Democratic Republic of Congo, where it caused a rare and dangerous hemorrhagic fever. While deadly, Ebola is only spread through direct contact with blood or other bodily fluids, making it very difficult to become infected. The 2014 outbreak of Ebola in West Africa is the largest in history, with over 22,000 people infected and 9,000 deaths. The CDC, the WHO, and many international health organizations formed a coalition to treat and to prevent spread of the disease.
- Chikungunya: This virus causes fever, muscle weakness, and crippling joint pain. Although it originated in Africa and Asia, the disease has spread to Europe and the Americas. As this virus is passed to humans via mosquitos, it can be prevented with insect repellant and clothing that minimizes skin exposure.
- Measles: The measles virus is highly contagious, infecting 9 of 10 people who are not immune. Within a few days of exposure, infected individuals will show symptoms of a respiratory infection with a flat red rash. About 3 in 10 people will develop serious complications like blindness and inflammation of the brain. Measles remains a public health concern in other parts of the world, as it causes 100,000 deaths per year worldwide. Due to widespread vaccination, measles was eliminated from the US since 2000. However, in 2019, doctors in the US diagnosed more than 1200 cases of measles – the most in more than two decades – because unvaccinated individuals are susceptible to the infection.
Edvotek provides several experiments to explore virology in your classroom laboratory: