What is a “virus”?
A virus is a strict parasite, meaning that it can only reproduce inside a host. Viruses do not contain the components of a normal organism like plants, animals or bacteria. Some say they are not even “alive” because they cannot reproduce without a host. To reproduce, they use their genes (encoded in DNA or RNA) to trick the host cell to use its own machinery to make more copies of the virus. For viruses to trick the host, they must enter the proper host cells. For example, SARS-CoV-2 can only reproduce within cells that line the human respiratory tract. A virus that causes gastro-intestinal disease like Norovirus, can only reproduce inside cells that line our intestines. Further, some viruses only infect certain bacteria or species of plants.
What's the difference between Enveloped and Non-enveloped viruses?
Regardless of their host, viruses can be classified based on their type of genetic material (DNA or RNA) and on their structure, like being enveloped or non-enveloped. The structure of all viruses includes a protein shell called a “capsid”. Enveloped viruses have an additional layer that covers the capsid. This membrane is composed of lipids and proteins it “stole” from the host cells and viral glycoproteins (sugars combined with proteins). The bumps, knobs and spikes that artists use in images of enveloped viruses like
SARS-CoV-2 depict structures on the viral envelope. These types of viruses need both an intact capsid and the envelope to infect cells. The envelope also helps avoid detection by the host immune system because it makes the virus look like just another host cell. But, the envelope also provides a soft target for destroying the virus when it is outside the host. Common disinfectants, and even alcohol, detergents or soap can disrupt the oily envelope and its components, destroying the ability for the virus to infect host cells.
Enveloped viruses can cause persistent infections and must transfer from host to host. Examples of enveloped viruses include ones that cause notorious diseases in humans, such as COVID-19, Influenza, Hepatitis B and C, and Hemorrhagic Fever (Ebola Virus Disease).
Non-enveloped viruses do not have a lipid covering, but their effects on humans can be just as devastating. These “naked” viruses only need their protein-based capsid and host detector proteins to infect host cells. However, because they lack a lipid envelope, they more resistant to many disinfectants and other stresses like drying out or heat. Examples of non-enveloped viruses include types that can cause dysentery (Norovirus), common colds (Rhinovirus) and Polio (Poliovirus).
Viruses are ubiquitous on Earth. While most do not cause harm except to their specific hosts, viral diseases can have devastating effects on entire populations of organisms, ranging from people to bacteria. One might think having an extra layer of “skin” might make enveloped viruses more resistant to environmental stress. While the envelope can help viruses infect cells once inside the host, it provides additional “machinery” on the outside the virus that can be broken with the right “monkey wrench”. Cleaning, sanitization and disinfection of surfaces is an important aspect in the ongoing battle against any microbial threats. But, for enveloped viruses like SARS-CoV-2, reducing the opportunity for person-to-person transmission through social distancing, hand-washing and proper personal protective equipment is the key to winning the war.