Chapter 14
Principles of Disease and
Epidemiology
Normal Flora (Normal Microbiota)
• Normal flora is found in respiratory tract,
gastrointestinal tract, genitourinary tract.
• Fetus growing the uterus has no normal
flora.
• Baby picks up the bacteria when the baby
comes through the birth canal.
• Usually Lactobacillus – first to colonize the
baby.
• As the baby starts breathing and drinking
more and more bacteria colonize the baby.
• Symbiosis – relationship between the
normal flora and the host.
• Commensalism – one of the organism is
benefited while the other is unaffected.
• Corynebacterium lives on the surface of
the eye. It gets nutrients from the sloughed
of tissue. It benefits form the host.
• It neither benefits nor harms the host.
• Mutualism – both organisms benefit from
each other. E. coli in the large intestine
makes vitamin – K. It gets shelter and
nutrients from the host.
• Parasitism – one organism is benefited at
the expense of the other. Pathogens such
as Mycobacterium tuberculosis are
parasites.
• Opportunists – part of the normal flora. Do
not cause problem when one’s immune
system is healthy. They can cause
infections such as UTI when one’s
immune system is weak.
Occurrence of the disease
• Sporadic disease – The disease shows up once
in a while in a population. Typhoid fever in USA.
• Endemic disease – The disease is constantly
present in a population. Common cold, malaria
in Africa.
• Epidemic Disease – Many people in a given
area get the disease within a short time.
• 50% of the population here get flu within a week
– flu is an epidemic disease.
• Pandemic disease – Epidemic disease
worldwide. AIDS is a pandemic disease.
• Transmission of the disease
• Contact transmission
Direct contact transmission – the
disease is transmitted by kissing,
touching, sexual contact
Indirect contact – transmitted by a fomite –
nonliving object – facial tissue, towels,
contaminated needle.
Severity and duration of the
disease
• Acute disease – develops rapidly and lasts
only for a short time – influenza
• Chronic disease – develops slowly and
lasts for a long time – tuberculosis
• Latent disease – microbe stays inactive for
a long time and then becomes active to
produce symptoms
Extent of host involvement
• Local infection – infection is limited to a small
area of the body – blisters, vesicles
• Systemic infection – microbes or their products
spread throughout the body – diphtheria
• Septicemia – growth of bacteria in the blood
• Primary infection – infection causing the initial
illness – influenza virus
• Secondary infection – caused by an opportunist
after the primary infection – Hemophilus
influenzae - peumonia
Source or reservoir of the disease
Humans – some people are carriers of
pathogens. They transmit the microbe
directly or indirectly to others. Typhoid
fever
Animals - some diseases are transmitted
from animals to humans – Lyme disease
Nonliving things – soil transmits fungal
spores, endospores of clostridium
Classifying infectious diseases
• Symptoms – subjective, not seen by the
observer, cannot be measured –
discomfort, pain
• Signs – can be seen by the observer and
measured. Fever, swelling
• Communicable disease – disease spreads
from one host to another – tuberculosis,
typhoid fever
• Contagious disease - disease that easily
spreads from one person to another –
plague.
• Non communicable disease – disease is
not spread form one person to another.
tetanus is caused by the bacterium in the
soil.
• Transmission of the disease
• Contact transmission
• Direct contact transmission – by kissing,
touching, sexual contact – Syphilis, genital
herpes
• Indirect contact transmission: by a
nonliving object – fomite – towels, facial
tissues, contaminated needle
• Droplet transmission: droplets released
during coughing and sneezing – cold,
influenza, diphtheria
• Vehicle transmission: by water, food and
air
• Waterborne: cholera – contaminated water
• Foodborne: undercooked contaminated
meat – tapeworm infestations.
• Airborne: fungal spores – histoplasmosis
• Vectors: insects
• Passive transmission: Pick up bacteria
from eye infection and release it on to
another person’s eyes.
• Biological transmission: insect bite can
introduce microbe – malaria - mosquito
Development of disease
• Period of incubation – period between the initial
infection and appearance of signs and
symptoms
• Depends on virulence, number of infecting
organisms, resistance of the host
• Prodormal period – characterized by early mild
symptoms – aches fatigue
• Period of illness – exhibits signs and symptoms
of the disease. Fever, chills, pain
• The immune response overcomes the pathogen.
• Period of decline – signs and symptoms
subside. The patient is vulnerable to
secondary infection by opportunists.
• Period of convalescence – the patient
regains strength and recovery has
occurred
• Nosocomial infections: infection one gets
when one stays in the hospital – caused
by opportunists such as E.coli,
Pseudomonas
• Epidemiology: A study of the source,
transmission, prevention
• 1848 John Snow – interviewed people
• People who drank from the Broad Street
pump got the disease.
• People who drank from other pumps and
people who drank only beer did not get the
disease.
• Broke the handle of the Broad St pump
• This dramatically reduced the number
cholera cases
• Source, transmission, prevention