Pathology is the study of disease, which occurs when pathogens like bacteria or viruses overcome the body's defenses. An infection results from the invasion and growth of pathogens in the body. Koch's experiments in the late 1800s established specific criteria known as Koch's postulates to determine the causative agent of infectious diseases. While useful for most bacterial diseases, there are some exceptions like diseases that have never been cultured. Once a microbe infects a host, disease develops through stages including incubation, prodrome, illness, decline, and convalescence.

1. Pathology,
Infection, and
Disease
Reference: Microbiology- An
Introduction,( Tortora, Funke, Case)
12th Edition

2. ● Disease-causing microorganisms are called pathogens. Pathogenic
microorganisms have special properties that allow them to invade the human
body or produce toxins.
When a microorganism overcomes the body’s defenses, a state of disease
results.
● Pathology is the scientific study of disease.
Pathology is concerned with the etiology (cause), pathogenesis (development),
and effects of disease.
● Infection is the invasion and growth of pathogens in the body.
● A host is an organism that shelters and supports the growth of pathogens.
● Disease is an abnormal state in which part or all of the body is not properly
adjusted or is incapable of performing normal functions.

3. The Etiology of Infectious Diseases
Koch was a German physician who played a major role in establishing that
microorganisms cause specific diseases. In 1877, he published some early papers
on anthrax, a disease of cattle that can also occur in humans. Koch demonstrated
that certain bacteria, today known as Bacillus anthracis, were always present in
the blood of animals that had the disease and weren’t present in healthy animals.
He knew that the mere presence of the bacteria did not prove that they had
caused the disease; the bacteria could have been there as a result of the disease.
Thus, he experimented further.

4. He took a sample of blood from a sick animal and injected it into a healthy one.
The second animal developed the same disease and died. These experiments
were reproducible.
Koch showed that a specific infectious disease (anthrax) is caused by a
specific microorganism (B. anthracis) that can be isolated and cultured on
artificial media. He later used the same methods to show that the bacterium
Mycobacterium tuberculosis is the causative agent of tuberculosis.

5. Koch’s postulates
Koch’s research provides a framework for the study of the etiology of any
infectious disease.Today, we refer to Koch’s experimental requirements as
Koch’s postulates. They are summarized as follows:
1. The same pathogen must be present in every case of the disease.
2. The pathogen must be isolated from the diseased host and grown in pure
culture.
3. The pathogen from the pure culture must cause the disease when it’s
inoculated into a healthy, susceptible laboratory animal.
4. The pathogen must be isolated from the inoculated animal and must be shown
to be the original organism.

7. Exceptions to Koch’s Postulates
Although Koch’s postulates are useful in determining the causative agent of most
bacterial diseases, there are some exceptions.
● Some microbes have unique culture requirements. The bacterium Treponema
pallidum is known to cause syphilis, but virulent strains have never been
cultured on artificial media. The causative agent of leprosy, Mycobacterium
leprae, has also never been grown on artificial media. In such cases Koch’s
postulates are modified to establish etiologies of diseases.
● Some diseases, such as tetanus, have unequivocal signs and symptoms but
some diseases, such as pneumonia and nephritis, may be caused by
a variety of microbes.

8. ● Some pathogens, such as S. pyogenes, cause several different diseases.
● Ethical considerations may also impose an exception to Koch’s postulates.
For example, some agents that cause disease in humans have no other
known host. An example is HIV, the cause of AIDS. This poses the ethical
question of whetherhumans can be intentionally inoculated with infectious
agents.

9. Development of Disease
Once a microorganism overcomes the defenses of the host, development of the
disease follows a certain sequence that tends to be similar whether the disease is
acute or chronic.
Figure: The stages of Disease.

10. Incubation Period
The incubation period is the interval between the initial infection and the first
appearance of any signs or symptoms. In some diseases, the incubation period
is always the same; in others, it is quite variable. The time of incubation depends
on the specific microorganism involved, its virulence (degree of pathogenicity),
the number of infecting microorganisms, and the resistance of the host.
Prodromal Period
The prodromal period is a relatively short period that follows the period of
incubation in some diseases. The prodromal period is characterized by early,
mild symptoms of disease, such as general aches and malaise.

11. Period of Illness
During the period of illness, the disease is most severe. The person exhibits
overt signs and symptoms of disease, such as fever, chills, muscle pain (myalgia),
sensitivity to light (photophobia),sore throat (pharyngitis), lymph node
enlargement(lymphadenopathy), and gastrointestinal disturbances. During
the period of illness, the number of white blood cells may increase or decrease.
Generally, the patient’s immune response and other defense mechanisms
overcome the pathogen, and the period of illness ends. If the disease is not
successfully overcome (or successfully treated), the patient dies during this
period.

12. Period of Decline
During the period of decline, the signs and symptoms subside. The fever decreases,
and the feeling of malaise diminishes. During this phase, which may take from less
than 24 hours to several days, the patient is vulnerable to secondary infections.
Period of Convalescence
During the period of convalescence, the person regains strength and the body
returns to its prediseased state. Recovery has occurred.
We all know that during the period of illness, people can serve as reservoirs of
disease and can easily spread infections to other people. However, you should also
know that people can spread infection during incubation and convalescence as well.