Viral Infections 101
COVID-19 has, understandably, generated a lot of interest in and questions about viruses. In this blog, we’ll talk about what viruses are, how they cause disease and spread, and what can be done to protect ourselves and our communities.
What are viruses?
A virus is a tiny germ. In fact, it’s one of the smallest agents that can cause an infectious disease. From a biological standpoint, viruses are very simple: they’re nothing more than a bit of genetic material inside a protein coating. One interesting fact about viruses is that questions remain as to whether they are alive. On their own, they don’t perform any of the processes that are regarded as essential for living things; for example, they are unable to reproduce.
There are a number of diseases caused by viruses. Some of these—the common cold, croup, or warts, for instance—are more troublesome than problematic. Of course, as COVID-19 has reminded us all too well, some viral diseases are quite serious. Viruses are also responsible for HIV/AIDS, smallpox, and Ebola.
How viruses cause infection
To reproduce, viruses must enter the cells of another living thing—a process we know as infection. And that’s where the trouble begins. Once a virus enters a cell, it takes over the machinery of the host cell and uses it to make more copies of itself. When the number of virus copies gets too big for the cell to hold, it bursts open. This kills the host cell and releases the newly formed viruses, which then invade more cells and continue the infection process.
How viruses spread
Generally, viruses spread through droplets that are expelled when air is released from the nose or mouth of an infected person, such as during coughing, sneezing, laughing, singing, and talking. These droplets fly through—and can remain suspended in—the air and thus can enter the mouths or noses of people nearby. The risk of passing the virus increases the more forcefully the droplets are expelled from the infected person and the closer and longer people are in contact. It is primarily these factors that drive the recommendation for maintaining 6 feet of space (that is, social distancing) between others when in public.
It is also possible for a person to introduce a virus into the body by touching a surface or an object that has been contaminated by droplets expelled from an infected individual and then touching his or own mucous membranes (for example, the eyes, nose, or mouth). It is for this reason that health experts strongly recommend frequent, diligent hand washing.
How our bodies fight viruses
Fortunately, there are obstacles a virus must overcome to enter the body and cause infection. The first challenge a virus faces is getting past the body’s physical barriers. The skin is the largest and most obvious barrier, but the virus must also get past the mucous membranes, which are the specialized layers that line the “key access ports” to the body, including the mouth, nose, and eyelids. In addition to providing a physical barrier to viruses and other infectious agents, the mucous membranes have other tricks up their proverbial sleeve. They secrete mucus, which traps invaders (think of it as the body’s flypaper) and contains antibodies that serve as the immune system’s advance guard.
Our bodies have ways of defending against viruses that make it past the skin and mucous membranes. Our immune systems produce antibodies and cells that are able to recognize and attack viruses or cells infected by viruses. How quickly and how thoroughly the immune system responds determines how sick we get. If the immune response is quick and robust, the virus may be destroyed before we even know we had an infection. If the virus gets too big of a head start, or if the immune system is not able to recognize or adequately fight the virus, we will become sick—or worse.
Viral infections in the community
For some viruses (including the coronavirus that causes COVID-19), there can be a considerable time (up to 2 weeks) between infection with the virus and the development of symptoms (this time is referred to as the latency period). This means that it is possible—and not uncommon—for individuals to unknowingly have the infection; such people are called asymptomatic carriers. Because they don’t feel sick, asymptomatic carriers may continue to interact with others, increasing the risk of passing the infection along. This pattern to a large extent explains how COVID-19 spread as easily and rapidly as it has in certain regions, accounting for what is referred to as community spread.
Another factor that contributes to how a virus spreads within a community is its seasonality. Some viral infections are more common during certain times of the year—think of fall and winter in the United States, a period that is commonly referred to as “cold and flu season.”
Why some viruses show seasonality while others do not is not fully understood. Some of the factors that could contribute to seasonality are how the virus is spread (if the virus is carried by an insect, for example, infections caused by the virus are likely to be more common in the seasons when the insect is most active and abundant) and their route of transmission (infections that spread through respiratory droplets will have greater opportunities to spread quickly when people are inside more often, such as during the colder months).
Preventing viral infections
For many (most?) viruses, the mucous membranes are the easiest access point to the body. Therefore, taking measures that minimize the risk of exposing the mucous membranes to viruses—like washing hands frequently; refraining from touching your eyes, nose, and mouth, avoiding crowds; and wearing masks when around others—are important.
For viral infections for which a safe, effective vaccine is available, vaccination is the best way to protect yourself. Vaccine development is a complex process that involves several stages of testing; historically, it has taken a number of years.
Another strategy that is sometimes used to prevent viral infections is the administration of immune globulins, which are agents that contain antibodies. The antibodies in immune globulins work the same way that antibodies produced by your own immune system work: they seek out and fight viruses. Immune globulins are administered intravenously and are have traditionally used for patients who do not produce a large enough number of antibodies to fight the infection on their own, either because the immune system is not working at full strength or because it has not yet encountered the virus so has not had the opportunity to develop defenses against it.\