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Human-Microbe Interactions
The scientific study of disease is called Pathology. Pathology is concerned with the structural and functional changes
Pathology involves the cause of disease (etiology) and the manner in which a disease develops (pathogenesis).
Infection is the invasion or colonization of pathogens in the body. An infection occurs when a pathogenic microbe is able to
multiply in the tissues where it lodges.
Disease is an abnormal state in which part or all of the body is not properly adjusted or is incapable of performing normal
functions.
An infectious disease results from the growth of a pathogenic microorganism.
A communicable disease is an infectious disease that can be transmitted from one person to another.
Example- Measles, diphtheria, gonorrhea.
Some diseases that are easily transmitted from person to person are also called contagious diseases such as common cold,
influenza.
A host is an organism that supports the growth of microorganism.
The ability of a pathogen to invade and infect the host, cause damage, and produce disease is termed its virulence.
Those pathogens that can cause disease relatively easily are called virulent pathogens; while microorganisms which do not
cause disease are called avirulent. Within a single species, some strains may be virulent while others avirulent.
Whether a disease results when a person is exposed to a pathogen depends on many factors which include hosts resistance
and the virulence of the pathogen.
Normal Micro-biota:
Animals and humans are usually germ free in-utero.
Microorganisms begin colonization in and on the surface of the body soon after birth.
Microorganisms that establish permanent colonies inside or on the body without producing disease constitute the normal
micro biota.
The transient micro biota is composed of microbes that are present for various periods and then disappear.
Relationship between the normal micro-biota and the host:
The normal microflora can prevent pathogens from causing an infection; this phenomenon is known as microbial antagonism.
Normal microflora protects the host from colonization by pathogenic microbes by competing for nutrients, producing
substances that are harmful to the invading microbes and changing the pH and oxygen availability.
The normal microflora and the host exist in a symbiosis.
Commensalism is a type of symbiotic relationship when one organism is benefited by the presence of other while the other
is unaffected. Most of the microorganisms that make up the normal micro flora are commensals
When both the organisms benefit by symbiosis it is called Mutualism.
When one organism is benefited while the other is harmed, the relationship is called Parasitism.
Opportunistic Microorganisms:
Some microorganisms do not cause disease under normal conditions. But when the host is weakened or compromised by
other infection, these microbes produce disease. These microbes are called Opportunistic pathogens. Example- tooth decay
and gum diseases are caused by normal micro flora of the mouth. AIDS is often accompanied by infection by Pneumocystic
carinii which causes pneumonia in these patients. This microbe does not cause disease in normal humans.
Some microbes may cause disease only when they colonize in certain region of the body. Neisseria meningitides often
found in the respiratory tract causes meningitis when it invades the brain.
Cooperation among Microorganisms:
Under some situations, one microorganism makes it possible for another to cause a disease or to produce more severe
symptoms. This is called cooperation.
Example- Presence of mycobacterium (tuberculosis causing bacteria) infections is known to increase the risk of acquiring
HIV infection and the infection with HIV induces spread of mycobacteria infection in the body. Thus mycobacteria
functions as cofactor in HIV infection.
Cofactors may be independent organisms or products of different organisms that synergistically enhance the disease.
Classifying Infectious diseases
Every disease that affects the body alters the body structure and function in particular ways. These alterations are
indicated in particular symptoms (subjective changes in body functions) such as malaise, pain etc. The patient also exhibits
certain signs which are objective changes that can be measured and observed such as swelling, fever, paralysis etc. A
specific group of symptoms or signs that accompany a specific disease is called a syndrome.
The occurrence of Disease
a. Disease occurrence is reported by incidence which is an index of the number of people contracting the disease.
b. The prevalence of a disease is the number of people having a specific disease at a specified time. Calculation of
incidence and prevalence rates helps scientists to estimate the range of disease occurrence and it's tendency to affect
some populations more than others. The frequency of occurrence is also a criteria used in the classification of diseases-
Sporadic disease: is a disease that occurs only occasionally. example typhoid fever in USA.
Endemic disease: A disease that is constantly present in the population; such a s common cold.
Epidemic disease: If many people in a given area acquire a certain disease in a short time
Pandemic Disease: An epidemic that occurs worldwide is called Pandemic.
The severity and duration of diseases:
Diseases are also classified on the basis of their severity and duration.
Acute disease is one that develops rapidly but lasts only a short time. example- influenza
A Chronic disease develops more slowly and the bodies reaction may be slow or less severe. But the disease is likely to
continue for a long time.
Example: tuberculosis, hepatitis B.
A disease that is intermediate between acute and chronic is called subacute disease.
Example: sub acute panencephalitis.
A latent disease is the one in which the causative agent remains inactive for some time and then becomes active to
produce the signs and symptoms of disease.
The rate, at which a disease spreads in a population and the number of people involved, is determined partly by the
immunity of the population. Natural infections by some microbes and vaccinations provide long lasting protection against
certain diseases. People who are immune to an infectious disease are not at risk of being reinfected and are thus not likely
to be carriers of that infection. Immune individuals thus would block the spread of infectious agents. Some times, people
are protected from contracting a communicable disease by not coming in contact with infected people. Immunity to an
infectious disease provided by vaccination prevents the rapid spread of the disease to some people in the population even
if they are not vaccinated. Herd Immunity (when many immune people are present in a population) protects the rapid
spread of infectious diseases.
Emerging Infectious diseases:
There is an increase in incidence of new or changing diseases caused by bacteria, viruses, protozoa or a helminth. These
diseases present symptoms that are different from the known diseases. factors contributing to the emergence of new
infectious diseases are:
•Widespread use of antibiotics and pesticides.
•Changes in the environmental conditions-global warming, has resulted in dissemination of diseases such as cholera, malaria,
Hantavirus disease.
•Known diseases spread to new geographic areas due to the modern transportation.
•Ecological changes brought about by deforestation, natural disasters, wars, result in spread of unrecognized infections.
Example-Spread of Venezuelan hemorrhagic fever among workers clearing South American forests.
•Some animal control programs have resulted in rising populations of animals that act as intermediate hosts for many
pathogens. The increase in Lyme's disease due to increasing population of deer is an example.
The scientific study of disease is called Pathology. Pathology is concerned with the structural and functional changes
Pathology involves the cause of disease (etiology) and the manner in which a disease develops (pathogenesis).
Infection is the invasion or colonization of pathogens in the body. An infection occurs when a pathogenic microbe is able to
multiply in the tissues where it lodges.
Disease is an abnormal state in which part or all of the body is not properly adjusted or is incapable of performing normal
functions.
An infectious disease results from the growth of a pathogenic microorganism.
A communicable disease is an infectious disease that can be transmitted from one person to another.
Example- Measles, diphtheria, gonorrhea.
Some diseases that are easily transmitted from person to person are also called contagious diseases such as common cold,
influenza.
A host is an organism that supports the growth of microorganism.
The ability of a pathogen to invade and infect the host, cause damage, and produce disease is termed its virulence.
Those pathogens that can cause disease relatively easily are called virulent pathogens; while microorganisms which do not
cause disease are called avirulent. Within a single species, some strains may be virulent while others avirulent.
Whether a disease results when a person is exposed to a pathogen depends on many factors which include hosts resistance
and the virulence of the pathogen.
Normal Micro-biota:
Animals and humans are usually germ free in-utero.
Microorganisms begin colonization in and on the surface of the body soon after birth.
Microorganisms that establish permanent colonies inside or on the body without producing disease constitute the normal
micro biota.
The transient micro biota is composed of microbes that are present for various periods and then disappear.
Relationship between the normal micro-biota and the host:
The normal microflora can prevent pathogens from causing an infection; this phenomenon is known as microbial antagonism.
Normal microflora protects the host from colonization by pathogenic microbes by competing for nutrients, producing
substances that are harmful to the invading microbes and changing the pH and oxygen availability.
The normal microflora and the host exist in a symbiosis.
Commensalism is a type of symbiotic relationship when one organism is benefited by the presence of other while the other
is unaffected. Most of the microorganisms that make up the normal micro flora are commensals
When both the organisms benefit by symbiosis it is called Mutualism.
When one organism is benefited while the other is harmed, the relationship is called Parasitism.
Opportunistic Microorganisms:
Some microorganisms do not cause disease under normal conditions. But when the host is weakened or compromised by
other infection, these microbes produce disease. These microbes are called Opportunistic pathogens. Example- tooth decay
and gum diseases are caused by normal micro flora of the mouth. AIDS is often accompanied by infection by Pneumocystic
carinii which causes pneumonia in these patients. This microbe does not cause disease in normal humans.
Some microbes may cause disease only when they colonize in certain region of the body. Neisseria meningitides often
found in the respiratory tract causes meningitis when it invades the brain.
Cooperation among Microorganisms:
Under some situations, one microorganism makes it possible for another to cause a disease or to produce more severe
symptoms. This is called cooperation.
Example- Presence of mycobacterium (tuberculosis causing bacteria) infections is known to increase the risk of acquiring
HIV infection and the infection with HIV induces spread of mycobacteria infection in the body. Thus mycobacteria
functions as cofactor in HIV infection.
Cofactors may be independent organisms or products of different organisms that synergistically enhance the disease.
Classifying Infectious diseases
Every disease that affects the body alters the body structure and function in particular ways. These alterations are
indicated in particular symptoms (subjective changes in body functions) such as malaise, pain etc. The patient also exhibits
certain signs which are objective changes that can be measured and observed such as swelling, fever, paralysis etc. A
specific group of symptoms or signs that accompany a specific disease is called a syndrome.
The occurrence of Disease
a. Disease occurrence is reported by incidence which is an index of the number of people contracting the disease.
b. The prevalence of a disease is the number of people having a specific disease at a specified time. Calculation of
incidence and prevalence rates helps scientists to estimate the range of disease occurrence and it's tendency to affect
some populations more than others. The frequency of occurrence is also a criteria used in the classification of diseases-
Sporadic disease: is a disease that occurs only occasionally. example typhoid fever in USA.
Endemic disease: A disease that is constantly present in the population; such a s common cold.
Epidemic disease: If many people in a given area acquire a certain disease in a short time
Pandemic Disease: An epidemic that occurs worldwide is called Pandemic.
The severity and duration of diseases:
Diseases are also classified on the basis of their severity and duration.
Acute disease is one that develops rapidly but lasts only a short time. example- influenza
A Chronic disease develops more slowly and the bodies reaction may be slow or less severe. But the disease is likely to
continue for a long time.
Example: tuberculosis, hepatitis B.
A disease that is intermediate between acute and chronic is called subacute disease.
Example: sub acute panencephalitis.
A latent disease is the one in which the causative agent remains inactive for some time and then becomes active to
produce the signs and symptoms of disease.
The rate, at which a disease spreads in a population and the number of people involved, is determined partly by the
immunity of the population. Natural infections by some microbes and vaccinations provide long lasting protection against
certain diseases. People who are immune to an infectious disease are not at risk of being reinfected and are thus not likely
to be carriers of that infection. Immune individuals thus would block the spread of infectious agents. Some times, people
are protected from contracting a communicable disease by not coming in contact with infected people. Immunity to an
infectious disease provided by vaccination prevents the rapid spread of the disease to some people in the population even
if they are not vaccinated. Herd Immunity (when many immune people are present in a population) protects the rapid
spread of infectious diseases.
Emerging Infectious diseases:
There is an increase in incidence of new or changing diseases caused by bacteria, viruses, protozoa or a helminth. These
diseases present symptoms that are different from the known diseases. factors contributing to the emergence of new
infectious diseases are:
•Widespread use of antibiotics and pesticides.
•Changes in the environmental conditions-global warming, has resulted in dissemination of diseases such as cholera, malaria,
Hantavirus disease.
•Known diseases spread to new geographic areas due to the modern transportation.
•Ecological changes brought about by deforestation, natural disasters, wars, result in spread of unrecognized infections.
Example-Spread of Venezuelan hemorrhagic fever among workers clearing South American forests.
•Some animal control programs have resulted in rising populations of animals that act as intermediate hosts for many
pathogens. The increase in Lyme's disease due to increasing population of deer is an example.