Professor Dave here, let’s talk about bacterial infections in humans.
By now, we’ve learned about the microbes that can be found in the world around us,
the general ways in which we interact with them, how to classify them, and how they cause
damage.
Now, let’s dig into what’s really happening on the human side of the equation, and how
these diseases are circulated.
There are several interchangeable terms we can use for diseases that spread from host
to host.
These are infectious diseases, contagious diseases, or communicable diseases.
But how do these diseases develop?
From the moment a microbe is introduced to a susceptible host to the moment that symptoms
begin to appear is known as the incubation period.
For diseases such as the common cold, the incubation period is typically a couple of
days.
But depending on the pathogen, the incubation time could be up to a few weeks, several months,
or even years, like in the case of leprosy.
The length of the incubation period depends on factors like the health of the host, the
growth rate of the pathogen, and the number of infectious cells the host was exposed to.
During the incubation period, the pathogen is multiplying at the port of entry, but hasn’t
caused enough damage to cause symptoms yet.
Immediately following the incubation period is the onset of illness.
In the early phases of some illnesses, you might experience what’s called the prodromal
phase, where you feel weak or achy before the full-blown illness comes on.
Next is the period of invasion, during which the pathogen reaches peak toxicity, multiplying
and establishing itself within the tissues.
Common symptoms include fever, cough, rash, diarrhea, swelling, pain, or loss of muscle
control, depending on the infection.
The period of invasion can vary drastically, depending on the pathogen.
Next, as the immune system fights off the infection, the illness progresses to the convalescent
period, which means a phase of recovery.
Some patients may stop taking antibiotics during this phase because they feel better,
but here’s why that’s not a good idea.
As your body is recovering and fighting an illness, there are still pathogenic bacteria
present, albeit at low levels, low enough to not cause noticeable disease.
Those few bacteria that are left are the most resistant of the bunch.
If you continue the full course of treatment, you take care of all of them, and recover.
If you stop taking the drug mid-course, those resistant bugs repopulate, stronger and more
resistant than ever.
Plus, that particular drug won’t work next time, and non one wants that.
Generally speaking, an infection is characterized by the progression of symptoms.
In an acute infection, like a standard case of strep throat, symptoms develop quickly,
and then clear quickly, and the person typically develops an immunity to reinfection with the
same pathogen.
In a chronic infection, such as tuberculosis, symptoms come on slowly and can last anywhere
from months to years.
In a latent infection, however, the illness never completely goes away.
In the case of syphilis, tuberculosis, or typhoid fever, for example, a person might
experience acute or chronic onset of symptoms, followed by a period of convalescence, then
latency, where there’s no sign of disease at all.
However, the disease can come back from dormancy at any time.
The pathogenic microbe might linger in host tissues without causing disease, waiting to
act when immunity is low.
Also worth mentioning are carriers, which are individuals that carry infectious diseases
in their tissues for months or even years, spreading them to other people without suffering
any signs or symptoms themselves.
Generally speaking, infectious diseases can be transmitted at any stage of the infection
cycle, it just depends on the microbe.
Some are more commonly contagious during the incubation period, while others, like Shigella
are more infectious during the invasive period.
Before we move onto detailed information about diseases, let’s get some more terms out
of the way.
Infections can be localized to a particular region in the body, like an ear infection
or a boil, or systemic, which means they affect the entire body, like chickenpox or measles.
Bacteremia implies that bacteria are circulating in the blood, while toxemia means that toxins
are circulating in the blood.
As you may have guessed, the suffix “-emia” means “in the blood”.
Septicemia is the term for large numbers of microbes multiplying and circulating in the blood.
Finally, some diseases cause sequelae, which means long-term or permanent damage.
You might be wondering how, then, do these diseases find us?
Infectious microbes have to have a “home base” of sorts.
This is called a reservoir, which is the natural habitat of a pathogen.
Reservoirs can be anything from human or animal carriers to plants, soil, or water.
Let’s say we have a living reservoir in the form of an animal harboring a pathogen.
This pathogen can be transmitted either directly from the animal to a human, through a vector
such as a flea, tick, or mosquito, or through a vehicle such as water.
Some pathogens, such as the causative agent of tuberculosis, can survive for great lengths
of time in nonliving reservoirs such as soil, air, or water, and infect an unsuspecting
human who comes in contact.
Asymptomatic carriers are, in fact, living reservoirs.
You might have heard of “Typhoid Mary,” one of the most notorious asymptomatic carriers
of all time, who unknowingly spread Salmonella typhi around New York in the early 1900s,
before we understood such phenomena.
Zoonosis refers to infectious agents that are indigenous to animals, but can be naturally
transmitted to humans.
In these cases, the human is called a “dead end host”, because they cannot transfer
the infection along.
However, zoonotic diseases account for almost 70 percent of emerging infectious diseases
around the world, especially affecting those who regularly work or live with animals.
To make sure we have it all straight, let’s talk about transmission.
Vertical transmission refers to the transmission of a pathogen from parent to offspring, either
through the placenta, milk, ovum, or sperm.
Horizontal transmission is the spread of a pathogen from one infected individual to another.
This can be direct, through skin contact, a bite from a vector, an injection, or exposure
to contaminated droplets, such as a sneeze.
This can also be indirect, from an infected surface, let’s say a garden hose or fork
that a sick person touched, or perhaps food, air, or soil.
A separate category that often gets overlooked is the oral-fecal route, in which feces is
ingested, either due to poor hand-washing during food preparation, or by touching a
surface that’s been contaminated with fecal matter and then touching your mouth.
Now that we’ve got all the language we need to understand diseases, let’s do a deep
dive into some of the most medically important diseases.