Coronaviruses are a diverse family of viruses known to cause respiratory illnesses in both animals and humans. First identified in the mid-1960s, these viruses are named for the distinctive crown-like spikes observed on their surface under an electron microscope; “corona” is the Latin word for crown. While many coronaviruses circulate among animals like camels, cats, and bats, they can sometimes evolve and cross the species barrier to infect people. In humans, these infections can manifest as mild ailments similar to the common cold or develop into severe, life-threatening diseases.
Classification of Human Coronaviruses
Seven types of coronaviruses infect humans and are categorized into two groups based on the severity of illness they cause. The first group consists of four common, or endemic, coronaviruses that circulate continuously: HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1. These viruses cause mild to moderate upper-respiratory tract infections, and most people will be infected with one of them at some point.
The second group includes three newer coronaviruses known for causing severe and widespread outbreaks: Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV), Middle East Respiratory Syndrome Coronavirus (MERS-CoV), and Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the virus responsible for COVID-19. Each of these viruses has a zoonotic origin. SARS-CoV was first reported in 2002, MERS-CoV in 2012, and SARS-CoV-2 in 2019.
Transmission and Viral Replication
Human coronaviruses primarily spread through respiratory droplets and smaller particles, called aerosols, generated when an infected person coughs, sneezes, sings, or talks. These particles can then be inhaled by people nearby. Transmission can also occur through contact with contaminated surfaces or objects, known as fomites, followed by touching the eyes, nose, or mouth.
Once the virus enters the body, it begins to replicate. The virus uses its spike proteins to attach to specific receptors on human cells, particularly in the respiratory tract. After binding, the virus enters the cell and releases its genetic material, a single-stranded RNA genome. The virus then hijacks the host cell’s machinery to produce copies of the viral RNA and proteins. These components assemble into new virus particles, which are then released from the cell to infect others.
Associated Illnesses and Symptoms
Infections with the four endemic coronaviruses result in mild to moderate upper-respiratory symptoms, similar to the common cold. Symptoms include a runny nose, sore throat, headache, and a low-grade fever. Most people recover from these illnesses within a short period with no lasting effects.
In contrast, infections with SARS-CoV, MERS-CoV, and SARS-CoV-2 can lead to more serious diseases. While initial symptoms may be similar to a common cold, they can progress to severe pneumonia with shortness of breath. In some patients, this can lead to acute respiratory distress syndrome (ARDS), where the lungs cannot provide the body’s organs with enough oxygen. These severe infections can also cause complications affecting the heart, kidneys, and other organ systems.
Diagnosis and Medical Interventions
Diagnosing a coronavirus infection is done through laboratory tests. The most common are molecular tests, such as the polymerase chain reaction (PCR) test, which identifies the virus’s genetic material from a respiratory sample. Another method is the antigen test, which detects specific proteins from the virus. These tests help to confirm an active infection.
Treatment for coronavirus infections depends on the severity of the illness. For mild cases, care is supportive, focusing on relieving symptoms through rest, hydration, and over-the-counter medications. Individuals with more severe disease caused by viruses like SARS-CoV-2 often require hospitalization. Hospital interventions may include supplemental oxygen, mechanical ventilation, and specific antiviral medications. Vaccines have been developed, particularly for SARS-CoV-2, to prevent severe illness.