Human health and disease immunity
Channel: ISHTA’s BIO ACADEMY
Date: 20/05/2023

A wide range of organisms belonging to bacteria, viruses, fungi,
protozoans, helminths, etc., could cause diseases in man. Such diseasecausing organisms are called pathogens. Most parasites are therefore
pathogens as they cause harm to the host by living in (or on) them. The
pathogens can enter our body by various means, multiply and interfere
with normal vital activities, resulting in morphological and functional
damage. Pathogens have to adapt to life within the environment of the
host. For example, the pathogens that enter the gut must know a way of
surviving in the stomach at low pH and resisting the various digestive
enzymes. A few representative members from different groups of
pathogenic organisms are discussed here alongwith the diseases caused
by them. Preventive and control measures against these diseases in general,
are also briefly described.
Salmonella typhi is a pathogenic bacterium which causes typhoid
fever in human beings. These pathogens generally enter the small intestine
through food and water contaminated with them and migrate to other
organs through blood. Sustained high fever (39° to 40°C), weakness,
stomach pain, constipation, headache and loss of appetite are some of
the common symptoms of this disease. Intestinal perforation and death
may occur in severe cases. Typhoid fever could be confirmed by
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Widal test. A classic case in medicine, that of Mary Mallon nicknamed
Typhoid Mary, is worth mentioning here. She was a cook by profession
and was a typhoid carrier who continued to spread typhoid for several
years through the food she prepared.
Bacteria like Streptococcus pneumoniae and Haemophilus influenzae
are responsible for the disease pneumonia in humans which infects the
alveoli (air filled sacs) of the lungs. As a result of the infection, the alveoli
get filled with fluid leading to severe problems in respiration. The symptoms
of pneumonia include fever, chills, cough and headache. In severe cases,
the lips and finger nails may turn gray to bluish in colour. A healthy
person acquires the infection by inhaling the droplets/aerosols released
by an infected person or even by sharing glasses and utensils with an
infected person. Dysentery, plague, diphtheria, etc., are some of the other
bacterial diseases in man.
Many viruses also cause diseases in human beings. Rhino viruses
represent one such group of viruses which cause one of the most infectious
human ailments – the common cold. They infect the nose and respiratory
passage but not the lungs. The common cold is characterised by nasal
congestion and discharge, sore throat, hoarseness, cough, headache,
tiredness, etc., which usually last for 3-7 days. Droplets resulting from
cough or sneezes of an infected person are either inhaled directly or
transmitted through contaminated objects such as pens, books, cups,
doorknobs, computer keyboard or mouse, etc., and cause infection in a
healthy person.
Some of the human diseases are caused by protozoans too. You might
have heard about malaria, a disease man has been fighting since many
years. Plasmodium, a tiny protozoan is responsible for this disease. Different
species of Plasmodium (P. vivax, P. malaria and P. falciparum) are
responsible for different types of malaria. Of these, malignant malaria caused
by Plasmodium falciparum is the most serious one and can even be fatal.
Let us take a glance at the life cycle of Plasmodium (Figure 8.1).
Plasmodium enters the human body as sporozoites (infectious form)
through the bite of infected female Anopheles mosquito. The parasites
initially multiply within the liver cells and then attack the red blood cells
(RBCs) resulting in their rupture. The rupture of RBCs is associated with
release of a toxic substance, haemozoin, which is responsible for the chill
and high fever recurring every three to four days. When a female Anopheles
mosquito bites an infected person, these parasites enter the mosquito’s
body and undergo further development. The parasites multiply within
them to form sporozoites that are stored in their salivary glands. When
these mosquitoes bite a human, the sporozoites are introduced into his/
her body, thereby initiating the events mentioned above. It is interesting
to note that the malarial parasite requires two hosts – human and
mosquitoes – to complete its life cycle (Figure 8.1); the female Anopheles
mosquito is the vector (transmitting agent) too.