Professor Dave again, let’s continue with virus-cell interactions.
Let’s review some basic terminology.
We’ve gone over certain terms as they relate to viral infection of bacteria, but let’s
make sure we understand each of them in the context of multicellular hosts as well.
To clarify, a host is an organism that supports the growth of a virus, which could be a unicellular
organism as we saw in the previous tutorial, but this can also mean animals, including humans.
An infection in this context refers to a virus that is successfully reproducing in a host.
And disease refers to an abnormal state of a host.
When a host cell and a virus encounter each other, our bodies’ barrier defenses and
immune systems, both innate and adaptive, work to prevent or control infection of the
organism as a whole.
Viruses cause disease after they break through the natural protective barriers of the body
and evade the immune system.
From there, the virus might trigger a destructive immune and inflammatory response, or it might
kill cells of an important tissue, like the brain, for instance.
Ultimately, the ability of the immune system to control the infection determines both the
duration and severity of the disease.
Now that we’ve got those basics down, let’s move on to some more involved aspects of a
virus-host interaction.
For any given virus, the host range refers to the spectrum of species that are susceptible
to an infection with that virus.
It turns out that most viruses have a pretty narrow range, and most viruses, but not all,
are able to infect only one species.
Now let’s think about this from the perspective of disease.
A disease can be caused by multiple viruses that have a common tissue preference, also
called a tropism.
For instance, viruses with a liver tissue tropism can cause hepatitis, or those with
a tropism for upper respiratory tract tissue can cause the common cold.
On the other hand, one virus might cause several different diseases, or no observable symptoms at all.
An example of this is herpes simplex virus, or HSV, type 1.
This virus can cause encephalitis, genital herpes, pharyngitis, or several other diseases
depending on what kind of tissue it infects.
Like bacteria, many viruses encode virulence factors that increase the efficiency of their
replication or transmission, the binding of the virus to its target tissue, or the immune
escape of the virus.
These factors may not be essential for growth of the virus in something like tissue culture,
but are necessary for survival or pathogenicity in a host cell.
If a virus loses its virulence factors, we refer to the virus as attenuated.
The portal of entry of a virus is often through mucous membranes, but viruses can also enter
through skin abrasions, insect bites, or other breaks in the skin, though it is less common.
In general, the skin serves as an excellent barrier to infection, and actually, inhalation
is probably the most common route of viral infection.
These infections can remain localized at or near the portal of entry, like in the case
of a respiratory infection.
Or, they might spread to a secondary target organ, which is called a disseminated infection.
An example of this is the measles virus, which enters in the respiratory tract but can spread
to other organs.
The extent of disease caused by a virus depends on a few factors, including: One, the nature
of exposure to the virus.
Two, the general health, age, and immune status, of the person.
Three, the viral dose.
And four, the genetics of the host and the virus.
Viral disease in the human body progresses through a series of defined steps, similarly
to how a virus enters and replicates in a cell.
Let’s take a look at these steps.
First, we have acquisition, or entry into the body.
The second step is initiation of infection.
After an infection is initiated, the next step is an incubation period, when the virus
is amplified and may spread to a secondary location.
From here, we see replication of the virus in the target tissue, which is what causes
the observable signs of disease.
Next up, our immune system responds, either limiting or contributing to the disease.
The virus is then produced and released, causing contagion.
Finally, the disease is either resolved, or continues as a persistent infection or chronic disease.
As with bacterial diseases, viral infections can be asymptomatic during the incubation
period, or they might cause nonspecific early symptoms, which we call the prodrome.
Ultimately, symptoms of a viral disease are caused by tissue damage or potential systemic
effects, either caused by the virus itself or by the immune system’s response.
To be clear, immune responses can have both positive and negative effects.
In some cases, the immune system may control the spread of the virus by eliminating the
virus and virus-infected cells from the host.
In such a case, the infection is resolved.
In other cases, however, viruses can cause immunopathology, where an immune response
is triggered that harms the host.
On the milder end of the spectrum, we might see simple inflammation, which just refers
to pain, redness, heat, and swelling caused by cytokines that are released by immune system
cells in response to the virus.
Context is critical here, though.
An acute, regulated, local case of inflammation is generally a good sign, while chronic, systemic
inflammation is more destructive and worrisome.
If the virus spreads, it might trigger a hypersensitivity reaction via our adaptive immune system, which
can be absolutely devastating.
The lymphatic system and bloodstream are the primary avenues for viral transfer within
the body once an infection is established.
The term viremia refers to the transport of viruses in the blood.
Viruses can also gain access to the central nervous system or the brain through several routes.
And finally, viral infection is considered resolved when all infectious viruses and virus-infected
cells are cleared from the body.
Now that we’ve got some information about how viruses move around the body of a host,
let’s move forward and talk about how viruses spread between hosts.