Professor Dave here, let’s spread some viruses.
If you are a living organism, you are constantly being exposed to viruses.
We typically encounter viruses in every aspect of our daily routine, though certain environments,
lifestyles, and living arrangements might increase the likelihood of coming into contact
with them.
For instance, a crowded living arrangement might increase your chance of being exposed
to a virus that causes something like respiratory illness or gastroenteritis.
Daycare centers and summer camps are also common sources of viral infections.
And, of course, working in a healthcare-related field will also increase your likelihood of exposure.
Let’s now get a closer look at precisely how viruses are transmitted.
The efficiency of transmission depends on a few things.
First, how much of the virus were you exposed to?
How infective is that virus?
How healthy are you?
And let’s say you encounter someone that is sick with a virus.
At what stage of infection did you encounter them?
All of these details are relevant.
We use the term “shedding” to refer to the spread of a virus to another organism.
It turns out that the most likely time for virus shedding to happen is before any symptoms
have shown up in the host.
This can vary from virus to virus, of course.
And while we’re at it, let’s quickly talk about the difference between generation time
and an incubation period.
The generation time is the average time between infection and transmission to a new host,
or, put differently, the time it takes a person to make enough virions before they can be
spread to another person.
The incubation time is the time from infection to the onset of clinical signs and symptoms.
Incubation time varies from person to person.
When it comes to routes of transmission, it really depends on the source of the virus
and the ability of that virus to withstand the environment it’s up against.
Zooming in on the molecular level, the presence or absence of a viral envelope is a primary
determinant for mode of viral transmission.
Non-enveloped, or naked, viruses can withstand extreme pH, detergents, and drying, whereas
enveloped viruses typically can’t.
Most non-enveloped viruses can handle the acidic pH of the stomach and detergent-like
bile of the intestines, which means that if someone infected with a non-enveloped virus
doesn’t wash their hands after using the bathroom, they could be unknowingly spreading
a virus that’s successfully made it all the way through their digestive system.
We refer to contaminated inanimate objects, like handkerchiefs, toys, or doorknobs, as fomites.
It might seem a little counterintuitive, but enveloped viruses are pretty fragile compared
to their non-enveloped counterparts.
Enveloped viruses require an intact envelope to be able to cause an infection.
In fact, they have to remain wet to be able to spread to begin with, so they’re typically
spread through respiratory droplets such as during a sneeze, or through blood, saliva,
or mucus.
They can also be spread by injection or organ transplants.
The most common routes of transmission into a human population are: either direct or indirect
contact with a non-human animal, or through a vector, like a mosquito, that acquired the
virus from a non-human animal.
On the other hand, for viruses already in a human population, the most common routes
of transmission are as follows.
One, the respiratory-aerosol route, or breathing in droplets from someone’s cough or sneeze.
Two, the fecal-oral route, or ingesting particles of infected feces.
Three, sexual transmission.
And four, the “other” category, including less common routes that use direct and indirect contact.
Transmission routes within human populations can be broken down further into horizontal
versus vertical transmission.
Horizontal transmission refers to the spread of a virus from one individual to another
non-related individual.
Vertical transmission means the virus is passed from parent to offspring.
Vertical transmission can occur via the placenta during birth, or through breast milk.
Similarly to how it works with bacteria, animals can serve as vectors that spread viral disease
to other humans and animals.
They can also be reservoirs, meaning they keep the virus maintained and amplified in
their environment.
And zoonoses refers to viral diseases that are shared by humans and animals.
Viruses that are spread by mosquito or tick are referred to as arboviruses, because they’re
arthropod borne.
Thinking to the population level, the ability of a virus to persist in a community hinges
on the availability of susceptible hosts, and the efficiency of viral transmission determines
the size of the susceptible population.
We use the term maintenance to describe the ability of a virus to keep going in a community.
We’ll cover more epidemiology in a later video, but for now, recall that the term epidemic,
or outbreak, refers to a frequency of cases that is above the normal baseline, or more
cases than were expected.
We’ll circle back to things like epidemiology and vaccines once we’ve explored some of
the most clinically relevant viruses one at a time, so let’s move forward and get started
with a survey of viruses.