Bacterial Infections in Humans


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.

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