Why Does Disease Spread so Fast?

Authors: Chad Cai, Jiayi Chen, Hill Huang, Charles Zhao, Lambert Zheng, Leon Zhou

Editors: Tiffany Chan, Joyce Hai, Demi Leng, Ethan Liu, Katelyn Ma, Lydia Ren, Shannon Tan, Tyler Vazquez, Molly Zhao

Artists: Tiffany Chen, Jane Liu, Nicole Wang, Nicole Wei

Different Transmission Types

To understand the fast spread of the current novel coronavirus, it is important to first introduce different transmission methods of other diseases. Common transmitting agents include sneezing, coughing, bodily contact, sharing needles, and fecal-to-oral contact. For instance, HIV requires direct sexual contact or shared bodily fluids (blood, seminal and vaginal) for others to be infected. This type of transmission makes HIV relatively difficult to spread in regular day-to-day contact. On the other hand, for COVID-19, respiratory droplets can be transmitted up to a distance of six feet from sneezing or coughing.

Although people tend to believe that the coronavirus cannot thrive without a host, a study from the New England Journal of Medicine found that viruses can still be infectious in the air or on surfaces for various periods, ranging from hours to days. When fluids, such as mucus and respiratory droplets, are left on a surface (doorknobs, pens, or credit cards), the virus can still cause infection. Another way that viruses spread is when people come in close contact with infected individuals. Schools, parties, and other gatherings have a high density of people, thereby increasing the risks of infection. The reason the virus spreads among people so rapidly is because of the asymptomatic "super shedders"; they are individuals who carry the disease but do not show regular symptoms (difficulty in breathing, fever, new loss of smell or taste, and vomiting) of COVID-19. Many of them have not been tested and carry the virus around with them without realizing they have it.

According to Dr. Jeffery Shaman from Columbia University, “these undocumented infections were about half as infectious per person as a documented case who has more severe symptoms and maybe shedding more. Undocumented infections drive the spread and growth of the outbreak”. Thus, one can catch COVID-19 from someone who has not been tested; someone who may be in perfect health can transmit SAR CoV-2 to others who are more vulnerable. These statistics and laboratory results serve critical roles in examining the rapid global spread of COVID-19.

Comparison of Transmission Methods of COVID-19 to Previous Infectious Diseases

The novel coronavirus, commonly known as SARS-CoV-2, carries its namesake due to its genetic relation to the virus that triggers Severe Acute Respiratory Syndrome, SARS-CoV-1. The transmission methods of SARS-CoV-1 are similar to those of the novel coronavirus. Both have spread among birds and mammals via respiratory droplets. Patients of SARS-CoV-1 can also infect others with the virus through close contact. If patients cough or sneeze without coverage, infection is likely to occur. However, according to Dr. Sandra Gonzalez Gompf, for SARS-CoV-1 to spread, people must be relatively close, around 3 feet, which is closer than the ubiquitous 6 feet of safe ¨social distancing¨ that has been announced to the world's populace. Moreover, SARS-CoV-1 can survive on metal, glass, and plastic for up to three days, according to a recent study published in the New England Journal of Medicine. This is remarkably similar to SARS-CoV-2. If one comes into contact with an object containing SARS-CoV-2 and touches their facial orifices, infection may occur. This is quite common among coronaviruses, especially SARS-CoV-2.

H1N1, otherwise known as the swine flu, is another infectious disease that can be compared to COVID-19. H1N1 can be transmitted if one comes into direct contact with swine; through zoonosis (transmitted to humans from animals), H1N1 may also spread to and among humans. However, H1N1 generally appears as a seasonal flu virus, which is less destructive than COVID-19; despite this, transmission methods are similar to the novel coronavirus. Both spread among people through respiratory droplets. One can also become infected if they touch objects that carry the H1N1 virus and then touch their mouth, nose, or eyes.

Transmission methods of these infectious diseases are quite similar, albeit making them difficult to detect early. Not only are they spread by respiratory secretions, but the incubation period of these diseases are relatively long as well. This makes distinguishing healthy people and asymptomatic people difficult without incessant testing.


Do not be misinformed. The coronavirus is a virus family, named after its crown-shaped protein shell. Seven coronaviruses exist in nature, yet among them, only three can cause severe dangers. While COVID-19 has been the most recent hot topic, another coronavirus that is relatively less mentioned in the Middle East Respiratory Syndrome (MERS).

Discovered in Saudi Arabia in 2012, the first major outbreak of MERS occurred in Western Asia. Studies have discovered that MERS was transmitted to humans from dromedary camels, camels with a singular hump. Similarly, all available evidence for COVID-19 indicates a zoonotic source, and its genetic resemblance to SARS suggests that its ecological origin is within the bat population.

MERS possesses a much weaker ability in regards to transmission rates. Only those who come into contact with a patient, such as a doctor without protective measures, possess the risk of infection. Thus, many cases of MERS were often clustered in medical care units. Meanwhile, COVID-19 travels from host to host extremely easily and in a variety of ways, such as through saliva.

The hauntingly plentiful methods in which COVID-19 spreads may account for the multitude of cases that have been identified worldwide. The number of cases soared in February and March, during which the pandemic began rapidly spreading in Europe and America. Yet, so far, 8 years after the first case, only 2,494 MERS cases have been confirmed by the WHO. On the other hand, the death rate among all COVID-19 cases is only around 5%, while the figure for MERS is over 30%.

There has been no effective cure developed for MERS thus far. The only thing doctors can do is to “support” the patients and hope that their immune system can eliminate the virus before cracking down. For COVID-19, it is almost the same. Most COVID-19 cases are mild or moderate, and many do not require hospitalization. In the meantime, remember to wear masks, wash hands frequently, and most importantly, do not panic.

How to Prevent Transmission

1. Droplet transmission: Coughing, sneezing, and talking are common droplet transmission methods. After someone is affected by the disease, their droplets, carrying the virus, can infect others. Droplet transmission is one of the most common ways for COVID-19 to propagate among people. Wearing masks can exhibit around 70% prevention for this type of transmission and masks are the most effective way to prevent this type of transmission.

2. Contact transmission: Touching certain facial orifices, when coming into contact with the virus, can transmit the virus. Coming into contact with surfaces and people are common contact methods. To prevent it, one should clean and disinfect all counters, knobs, and other surfaces they touch several times a day. Making sure not to touch facial orifices such as the mouth, eyes or nose after touching surfaces and others can help prevent transmission as well.


Aerosol propagation: Aerosol propagation refers to the fact that droplets gradually lose moisture during air suspension, forming droplet nuclei. If droplets carry the novel coronavirus, it can also cause infection via inhalation. As the propagation distance increases, the virus' content in the air gradually decreases, reducing the possibilities for people to be infected through this method. Like stated above, masks are an effective prevention method.

4. Fecal mouth infection: Fecal-oral transmission is another transmission type for COVID-19. Studies suggest that the virus can be found in infected individuals’ feces. If one uses the bathroom and does not wash their hands before coming into contact with food or others, they could unknowingly spread the disease to others. Be sure to wash your hands after using the bathroom, especially in public areas.

The virus mostly spreads through people who show symptoms. It is also possible to pass the virus without showing any signs. Individuals who do not know that they have been infected and do not display any symptoms can still transmit the virus to others. This is called asymptomatic spread. To avoid this, one should aim to stay at home, and when going out, avoid making direct contact with others through social distancing.

Transmissions Among Humans and Animals

It is important to note that while most viruses can easily spread between humans, COVID-19 is zoonotic, meaning that it can be transmitted from animals as well.

SARS-CoV uses a human receptor, called Angiotensin Converting Enzyme 2 (ACE2). This protein is often useful in lowering blood pressure, but it is found as an entry point for the virus to enter healthy cells.

Recent research has also discovered that some animals can be infected by COVID-19. There is no certain evidence to show how these animals become infected, but there are reasons to believe that COVID-19 can spread among animals.

The basic principles of COVID-19 affecting humans seem to be clear, but for the disease to be spread zoonotically, the intermediate host is also a vital factor. Generally speaking, viruses have different adaptability in different species, and zoonosis is hindered by various factors. In the case of bat-to-human transmission, the presence of an intermediate host increases the feasibility for the virus to spread between the two species, despite their significant developmental differences.

As a result, when discussing the methods in which COVID-19 propagates zoonotically, information on intermediate species is indispensable. On February 7, 2020, South China Agricultural University released its analysis on the virus genome, and found that the sequence of the virus strain isolated in the pangolin is 90.3% similar to the new coronavirus strain currently infecting humans. As a result, the pangolin is likely to be the COVID-19 intermediate host. Some studies also claim that minks, sea turtles, snakes, and salmons may also be the potential intermediate hosts for COVID-19.

How Do Diseases Become a Pandemic

Pandemics, according to their classical definition, are epidemics that cross international boundaries and affect a large number of people worldwide.

Global health crises tend to grow in phases. This chain of events starts with an “outbreak”—a sudden rise in confirmed cases of a disease, contained in a small geographic region. If the disease spreads beyond that initial region, it will become an epidemic. Apart from the various transmission types of COVID-19, the inefficiency of detecting the virus in its initial outbreak has substantially contributed to its spread. On December 30, 2019, a Chinese doctor wrote a letter to his superiors, warning them of a patient diagnosed with SARS in Wuhan—it was later confirmed that this was the first known case of COVID-19. People traveling from Wuhan aided in the spread of the virus both in and out of China, making it difficult for the government to contain the disease within China.

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