In early 2020, COVID-19 burst upon the worldwide scene, spreading rapidly through a completely susceptible population. It is the fourth coronavirus outbreak to occur since 2000, alarming many public health experts. COVID-19 is thought to be a “spillover” event in which a pathogen present in animals finds a way to infect humans.

There are seven coronaviruses that infect humans, most causing only mild symptoms. Recent outbreaks of more virulent and deadly coronaviruses include SARS-CoV-1, MERS, and SARS-CoV-2 (COVID-19). These three pathogens cause more severe symptoms and can lead to death in compromised individuals. At this point in the pandemic, treatment is supportive with no vaccine yet available.

As we face the dire effects of this pandemic, it is important to understand how a pathogen infects a susceptible population. This is best described using the concept of a chain of infection in which a pathogen moves from one link to the next until the chain is broken. If the chain is broken—using public health measures—a pathogen is less likely to achieve its goal of infecting more individuals.

Understanding how many people in a susceptible population are infected and how easily a pathogen spreads from person to person is calculated using a mathematical concept that produces what is referred to as the basic reproduction number of a disease (called R naught). Although determining the R naught for an infectious disease is fraught with difficulties, it nevertheless gives public health officials and broad idea of how transmissible a disease is and, in the absence of effective public health measures, how many people may become infected.

R₀ is one of the most widely used metrics in epidemiology to determine how far and how easily a disease spreads. Called the basic reproduction number, it is an indicator of the contagiousness or transmissibility of infectious and parasitic agents and represents the number of new infections estimated to stem from a single case in a population that has never seen the disease before.

R₀ is one of the key values that can predict whether an infectious disease will spread into a population or die out. It is used to assess the severity of the outbreak, as well as the strength of the medical and/or behavioral interventions necessary for control.

If the R₀ of a disease is 2, then each infected person will spread the illness to an average of 2 people. A disease with an R₀ below 1 is expected to eventually die out while an R₀ of 1 means a disease will remain stable in a population but will not cause an epidemic. If the R₀ is greater than 1, an epidemic may occur.

In the hands of experts, R₀ can be a valuable concept. However, the process of defining, calculating, interpreting, and applying R₀ is far from straightforward. When used by those who are not trained, R₀ is easily subject to “misrepresentation, misinterpretation, and misapplication”.

Herd immunity threshold for a given virus as the percentage of the population that must be immune to ensure that its introduction will not cause an outbreak. Many people erroneously think that once a large percentage of the population is infected with a pathogen, there is no longer a threat of infection. There are a number of reasons why this is an ineffective strategy: it will lead to many more deaths than would occur if public health measures are widely adopted, it fails to protect vulnerable segments of the population, and during a pandemic, herd immunity will not stop the spread of a disease.

Testing is critical for the identification and tracking of COVID-19 infections. Diagnostic tests, such as antigen tests and molecular diagnostic tests, are used to help with the diagnosis of COVID-19. Antibody tests can be used to detect the presence of antibodies in a person who has recovered from a bout of COVID-19.

Currently, there are several vaccines for COVID-19 with others in various stages of testing and trials. There are a number of anti-viral, immune-based, and adjunctive therapies in development that have shown promise in treating some of the worst symptoms of the virus.

Even with a vaccine, tried and true public health measures such as contact tracing, social distancing, masks, and handwashing continue to be recommended. Many in the U.S. are ignoring these important public health measures, which have the power to greatly diminish the spread of the virus if widely followed.

For historical guidance, we can look to successful public health responses during past pandemics and epidemics. Since 2000, three major outbreaks have occurred involving coronaviruses. The first two were contained without blossoming into pandemics. We are in the third outbreak now.

There is no doubt that this pandemic has severely affected poor and minority communities. Across the globe, poverty, lack of access to healthcare, food and water insecurity, and lack of coordinated government leadership has had disastrous consequences.