What Is a Virus?
Although viruses seem to display goal-directed behavior (invading cells and multiplying), they are not living things. To be classified as something alive, an entity must be made of cells; cells which metabolize nutrients, grow and divide. Viruses aren’t made of cells and don’t engage in any of these life processes.
Viruses are acellular particles made of at least a molecule of nucleic acid (genetic material) and a protein capsid (a container for the genetic material). Viruses are strictly parasites. They cannot reproduce without infecting a living cell and turning that host cell into a little virus factory. Viruses are never considered normal flora. They provide no benefit to the host cells that they occupy.
Viruses infect living cells and once inside, transform the cell essentially into a factory for making more viruses. These acellular particles are composed of nucleic acid (genetic material), proteins and, in some cases, lipids as well.
Viruses reproduce via four basic steps. Viral reproduction includes:
1. Delivery of the viral genetic material into a host cell
2. Commandeering of the host cell transcription and translation machinery
3. Use of the host cell’s building blocks to copy viral genomes and synthesize viral proteins
4. Viral genomes and proteins then self-assemble and exit host cells as new infectious particles.
Viruses exist in one of two states; extracellular and intracellular.
Extracellular State: Before it invades a host cell, a virus is in the ‘extracellular state’. An extracellular virus, called a virion (vie-ree-on), consists of a protein coat (capsid) surrounding nucleic acid. In addition, some viruses have phospholipid envelope surrounding the capsid.
Intracellular State: Once the virus invades a host cell it is in an ‘intracellular state.’ In this state, the capsid is removed and the virus exists as only as nucleic acid (genetic material).
What Is a Viroid?
Viroids are smallest known agents of infectious disease. Whereas viruses are made up of nucleic acid encapsulated in protein (capsid), viroids are uniquely characterized by the absence of a capsid. Thus far, this type of acellular particle has only been identified as an infectious agent in plants.
What Is a Prion?
Prions are an abnormal form of a normally harmless protein that cause various fatal neurodegenerative diseases called Transmissible Spongiform Encephalopathies. Once present in the brain, prions cause normal proteins to refold into abnormal shapes, destroying neurons and eventually causing the brain to become riddled with holes.
Bacteria and viruses are often lumped together into a general category of “bad things that can cause infection”. But these two types of infectious agents are very different, and the differences are important to your health.
Bacteria are living single celled creatures. In contrast, viruses are not alive. So to clearly understand the difference between bacteria and viruses, it is important to first know what exactly distinguishes life from non-life.
What Is a Cell?
All living things are made cells. These tiny units are the basic building blocks of life, and all living organisms are composed of at least one (unicellular) or more (multicellular) cells. A cell has all of the equipment that it needs to grow, turn food into energy (metabolize), divide and make more cells.
There are only two main types of cells; eukarotic and prokaryotic. Eukaryotic cells (a.k.a. eukaryotes) are the cell type that we are generally more familiar with. Humans, along with other animals, plants, fungi, protozoans and a few other oddities are all eukaryotes. Eukaryotic cells are larger, more evolutionarily recent and more complex than prokaryotic cells. One of the most visible structures of eukaryotic cells, absent in prokaryotes, is a nucleus; genetic material (nucleic acid) enclosed by a membrane.
Bacteria are prokaryotic cells, which are typically smaller, simpler and are more evolutionarily ancient that are eukaryotes. These tiny life forms are nearly always single celled and are all classified as either bacteria (Eubacteria) or bacteria-like (Archaea) organisms.
All bacteria are prokaryotic living cells, but all aren’t bad guys. Our bodies are covered with many different species of bacteria, considered our normal flora. Normal flora help to crowd out pathogens (“bad guy’” microbes) and some even produce materials, such as vitamin K, that help us stay healthy.
Pathogenic bacteria are the ones that typically make us sick; some can even cause infections that kill. For example, Clostridium botulinum is the causative agent of botulism. This bacterium produces a potent toxin called botulin that can block nerve function. Botulinium toxin is one of the most poisonous naturally occurring substances in the world. Other examples of pathogenic bacteria that are frequently in the news include MRSA (a.k.a. methicillin resistant Staphylococcus aureus), Clostridium difficile (a.k.a. C. diff), and Vibrio cholera, the microbe that causes cholera outbreaks.
Medications for Viral Versus Bacterial Infections
Sometimes, if you’ve come down with an infection, and go to the doctor seeking pharmaceutical relief, your physician won’t write a prescription for antibiotics. This is because antibiotics kill bacteria (living prokaryotic cells) but have no effect on viruses. Antibiotics are selectively toxic, they target either structures or metabolism specific to prokaryotic cells. This selective toxicity is why antibiotics don’t harm human cells, but can still kill bacteria.
The common cold, influenza and a whole host of other illnesses are caused by viruses, and antibiotics have no effect on them. Most of the prevalent, non-life-threatening viral illnesses just need to run their course. For some of the more serious viral infectious diseases, there are vaccines to prevent infection such as those that prevent polio, or chickenpox and there are antivirual medications to lessen the impact of infection.
What Is a Cell?
Living things are constructed of cells and can be unicellular (one cell) or multicellular (many cells).
Limits on Cell Size: Cells size is limited because cells must be able to exchange materials with their surroundings, and surface area relative to the volume decreases as size of cell increases. This limits the size of cells.
Cell Theory: The basic rules that apply to these smallest units of life state:
All organisms are composed of one or more cells.
Cells are the basic unit of structure and function in organisms.
All cells come only from other cells.
Infectious disease can be the result of cellular organisms, such as bacteria (prokaryotes), from eukaryotes (cells like ours) or from nonliving infectious agents such as viruses, virioids and prions. Here is a summery of the different types of acellular, nonliving infectious agents.
Acellular Particles: Viruses, Viroids & Prions
Although they may seem to behave like living things, acellular particles are not alive.
are not made of cells
cannot reproduce on their own
do not grow or undergo division
do not transform energy
lack machinery for protein synthesis
are so small that they can only be seen with an electron microscope shown below.