The ability of a microorganism to cause disease is
called pathogenicity. There are several pathogens that can cause serious harm
or even immediate death.
Invasion and multiplication of pathogenic
microorganisms in the body is called an infection. When we are infected by
pathogens we become sick, which means that our bodies stop functioning properly.
Infectious agents, such as bacteria, a virus, fungi or protozoa
causecommunicable diseases. Communicable diseases can be spread from one person
to another.
Infection transmission
All living organisms have a natural or acquired
resistance mechanism called immunity. When we get sick, for example, we use
different body cells and chemicals to fight bacteria. Bacteria in their turn
use different chemicals to fight us. That is why infection is sometimes
referred to as a race between pathogen and host organism. The infection can be
transmitted by direct or indirect contact.
Direct contact transmission - involves any direct
contact with an infected individual. Infection can be passed in water droplets
through a sneeze, cough, laugh or exhalation and through bodily fluids. Most
communicable diseases like colds, influenza, tuberculosis and HIV are spread
from person to person through infected fluid droplets.
Indirect contact transmission - is a method of
spreading infection from person to person that involves contact with a
contaminated object. Objects can become contaminated when touched by someone
with an infection. The infected object is called a fomite.
Another form of infection transmission through
indirect contact is through the oral-feacal route, which usually involves
ingesting contaminated water.
A third method of indirect contact involves
vector-borne diseases, which are carried by animals and insects. A vector is an
organism that serves as a ’go-between’ in the transmission of a host-to-host
disease.
Examples of diseases that can be transmitted via
indirect contact are cholera, Salmonellosis and dysentery.
Types of pathogens
Natural or human-triggered changes in the
environment might upset the natural balance between living organisms. These new
environmental conditions may encourage pathogens, allowing them to multiply
rapidly and increase the risk of exposing humans who share that environment.
Here are the main groups of human pathogens with some examples of the diseases
they cause.
Bacteria
E.coli causes food poisoning and urinary tract
infections.
Mycobacterium tuberculosis causes tuberculosis.
Viruses
Influenza virus causes ’flu’.
Herpes simplex virus causes herpes.
Protozoa
Plasmodium causes malaria.
Fungi
Tinea causes ringworm.
See animation 1.
Epidemiology
Epidemiology is the study of the patterns, causes
and prevention of human diseases. An outbreak of a contagious disease that
spreads rapidly and widely is called an epidemic. An epidemic occurring over a
very large area is called a pandemic. The term ’epidemiologic triangle’ is used
to describe the intersection of host, agent and environment in analysing an
outbreak.
The prevalence of a disease is the number of
diseased individuals at any one time (point prevalence) or over a given period
(period prevalence). The incidence is the number of new cases of a disease that
occur within a defined population over an established period of time.
History of epidemics
The influenza (virus-caused) pandemic of 1918-1919
killed more people than World War I. It is considered the most devastating
epidemic in recorded world history. More people died of influenza in a single
year than in four years of the Black Death Bubonic Plague from 1347 to 1351.
Bubonic plague is caused by bacteria called Yersinia
pestis. These bacteria live in fleas on rats. Rats were abundant in medieval
times due to very low hygiene standards, and the lack of sewage systems and
hot, running water. Bubonic plague killed about a quarter of the European
population of that time. Today this disease can be controlled by antibiotics
but outbreaks sill occur sometimes. See image 3.
Prevention of infections
The best way of fighting disease is to prevent it.
The discovery and manufacture of antibiotics and vaccines in the early part of
the 20th century have changed the state of people’s health forever. There are
three stages of prevention of infections: primary, secondary and tertiary.
The Primary stage - involves public education about
infectious diseases.
The Secondary stage - involves treating the actual
infection that has already occurred by quarantining and/or vaccinating of
infected individuals.
The Tertiary stage - involves the recovery from
illness.
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