Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

NEET: Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

The document Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
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What are Diseases?

A disease is an abnormal condition affecting a healthy living organism. It is broadly divided into infectious and non-infectious.

Infectious diseases - These diseases are caused by the pathogens, such as bacteria, virus, fungi, parasites and can be easily transmitted from one person to another, hence it is also known as a contagious or communicable disease. Common Cold, Tuberculosis, flu, ringworm, malaria are some examples of infectious diseases.
Non-infectious diseases - Diseases which cannot be transmitted from one person to another are called non-infectious disease, it is also known as a noncommunicable disease. These diseases can be either caused by genetic disorders, unhealthy diets, lack of physical activity, excessive use of tobacco, drugs or alcohol and few environmental factors.

Some Common Diseases In Humans

  • A pathogenic bacterium known as Salmonella typhi is known to cause typhoid in humans. This fever can be confirmed by Widal test
  • Pneumonia is caused by Streptococcus pneumoniae and Hemophilus influenza
  • Rhinovirus, a group of virus, is known to cause one of the most of infectious ailments in humans, cold.
  • Plasmodium, a small protozoan causes malaria. Another protozoan, Entamoeba histolytica causing amoebiasis (amoebic dysentery)
  • Ascaris(an intestinal parasite) causes ascariasis
  • Wuchererta, the filarial worm causes filariasis or elephantiasis


Immunity is defined as the ability of the body to protect, defend and fight against invading pathogens like bacteria, virus, and other foreign bodies and toxic substances. There are two types of immunity.

  • Innate immunity – non specific defence type of immunity present at the time of birth achieved by the provision of 4 barrier types – the physical barrier, physiological barrier, cellular barriers, cytokine barrier
  • Acquired immunity – pathogen specific immunity that is characterized by memory.

Active and Passive Immunity

Active immunity is where the host produces antibodies in the form of dead or living microbes when it is exposed to antigens. It is a slow process, taking time to provide its full effective response. Passive immunity, on the other hand, is the immunity where ready-made antibodies are given directly to protect the body against any foreign agents.


  • Acquired Immuno Deficiency Syndrome is caused by HIV(Human Immunodeficiency Virus) a member of the retrovirus group.
  • It is generally transmitted through sexual contact with infected person, blood transfusion with contaminated blood and its products, through the share of infected needles, from the infected mother to her child through the placenta. ELISA(Enzyme-linked immunosorbent assay)  is the diagnostic test for AIDS


Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

What are Pathogens? 

A large variety of entities exist. Some can thrive and grow inside the host. If the organism can inflict infectious diseases, it is an infectious agent. Manual infectious agents that can cause illness and infection are referred to as pathogens.

Classification of Pathogens 

Pathogens are classified into four main categories concerned with the classes of pathogenicity relying on general risk analysis and assessment available under the purview of the present practical and theoretical frameworks. These include:

Class 1

One of the below criteria has to be covered by the microbial species like:

  • They should not be part of an identified group of disease-causing agents in animals or humans.
  • They had proven to be safe in the past history under conditions, without any sort of restrictions physically despite taking into account that species have a difference in virulence between strains within them.
  • The species that belong to one of the below classes, but a particular group of the chain might have fewer genes that could create pathogenicity in animals and humans. The species of a particular chain might be put in an exceptional position depending on the degree of attenuation.

This would imply that out of categories 2, 3 and 4 species, the certain chain would be downgraded to a class.

  •  Both in vivo and in vitro testing, the non-pathogenicity of the species has been demonstrated authoritatively.

Class 2 

  • Species which spread diseases in humans or animals, might not spread to the humans and that has necessary prophylaxis or therapy.

Class 3 

  • Species which cause serious disease in humans,  spread to the humans while having the necessary prophylaxis or therapy.

Class 4

  • Species that cause serious human disease, which is likely to disseminate in the human population and for which no adequate prophylaxis or therapy exists.

What is Malaria?

Malaria is a mosquito-borne infectious disease caused by various species of the parasitic protozoan microorganisms called Plasmodium. Malaria is a disease that man has battled with for a long time. The first evidence of this protozoan came from mosquitoes preserved in amber nearly 30 million years ago.
It is even thought to have brought the Roman Empire to its knees. Malaria was so prevalent during the Roman times that the disease is also called ‘Roman Fever’. Today, the credit for actually discovering the parasite is given to Charles Louis Alphonse Laveran, a French physician. He even won the Nobel Prize in 1907 for his findings.
Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

Causes of Malaria

There are many factors that can cause malaria, such as –

  • Bitten by a malarial vector (Anopheles stephensi)
  • Use of shared and infected syringes.
  • Organ transplantation.
  • Transfusion.
  • From an infected mother to her baby during birth.

Symptoms of malaria

Symptoms of malaria are exhibited within 7 to 18 days of being infected. Common symptoms include:

  • Fever, fatigue, chills, vomiting, and headaches
  • Diarrhoea, anaemia and muscle pain
  • Profuse sweating and convulsions
  • Bloody stools.
  • In severe cases, malaria can be devastating; it can lead to seizures, coma and eventually, death.

Sir Ronald Ross and his study on the transmission of the disease helped carve the way for future scientists to effectively combat the disease. His deep research showed that specifically, the female Anopheles stephensi mosquito is the vector of the disease, and addressing this problem will prevent malaria and in turn, save countless lives.

Malaria Life CycleCommon Diseases in Humans - 1 Notes | Study Biology Class 12 - NEETMalaria parasite exists in the form of a motile sporozoite. The vector of malaria i.e. the female Anopheles mosquito transmits the malarial sporozoites into the hosts. When an infected mosquito bites a human, the sporozoites are injected into the blood through the mosquito’s saliva. The sporozoites travel into our body and accumulate in the liver. These parasites initially multiply within the liver, by damaging the liver and rupturing the blood cells in the body. Malaria kills by causing the destruction of the red blood cells in the host. The parasites reproduce asexually in the RBCs, bursting the cells and releasing more parasites to infect more cells. The rupture of red blood cells by the malaria parasite releases a toxin called hemozoin which causes the patient to experience a condition known as the chills.
When the female Anopheles mosquito bites an infected human, the parasites enter the mosquito’s body along the human blood it is drinking. It is inside the mosquito’s body that the actual development and maturing of the parasite happens. The parasites produced in the human body reach the intestine of the mosquito where the male and females cells fertilize each other to lead to the formation of a sporozoite. On maturing, the sporozoite breaks out the mosquito’s intestine and migrate to the salivary glands. Once they reach salivary glands, they wait till the mosquito bites another human and the process of infection and disease begins all over again. It is prudent however to observe that the complete development of the malaria parasite takes place in two different hosts; humans and mosquitoes.

Prevention of malaria

Malaria is one of the major causes of preventable death in the world today. It affects more than 500 million people worldwide and causes 1 to 2 million deaths every year. It is a tropical infectious disease and almost 90 per cent of the cases are from Sub-Saharan Africa.
There are two ways to deal with malaria – prevent the mosquito bite from happening (i.e preventative steps) or attack the parasites once they have infected the body.
The first method advocates the use of mosquito nets and mosquito repellent such as permethrin to prevent mosquitoes from biting. The second form of treatment uses a chemical called Quinine present in the bark of a cinchona tree. A form of drug chloroquine has proven very effective against malaria even though it is not a vaccine.


Dengue Fever

Dengue is a mosquito-borne viral disease caused by the Dengue virus. In this case, the dengue virus is transmitted by female mosquitoes – Aedes aegypti. These dengue mosquitos generally bite during the daytime and are found everywhere (Both inside and outside the house). These mosquitos are found to be at the peak of their activeness at dawn and dusk. The symptoms can develop only after 6 to 10 days after bitten by an infected mosquito.
Dengue fever is transmitted by mosquitos which carry the dengue virus, which has four varied serotypes to infect human beings. The serotypes mentioned above denotes a set of microorganisms that are exceptionally closely associated. These microorganisms can only be distinguished due to them having somewhat dissimilar antigens (the alien unit that affects the body and making us produce antibodies) which prompt the body to create some dissimilar antibodies. Dengue cases are more common in subtropical and the tropical regions of our planet, including our country.

Signs and Symptoms of Dengue Fever

Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

Dengue has an unexpected attack, viz. a sudden start and these symptoms could be an indicator of its onset.

  • Loss of appetite.
  • Diarrhoea and vomiting.
  • Gum and nose bleedings.
  • Severe joint and muscle pain
  • Fatigue, Nausea, and Vomiting.
  • A sudden drop in blood pressure.
  • Multiple rashes and wounds on the skin.
  • Pain behind the eyes coupled with extreme headaches.
  • The patient might feel week with high fever for 3-7 days.

Diagnosis of  Dengue Fever

The presence of the Dengue virus in the blood cells can be diagnosed by isolation of the virus, testing serum samples, and by other molecular methods. A patient with this syndrome is allowed to have a few blood tests to check the total count of red blood cells, blood platelets, and other physical examinations are conducted by the physician to evaluate that the symptoms are caused by a dengue infection.

Treatment for Dengue Fever

Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

Till today, there is no definite treatments or specific medicine to treat dengue infection. In general, the doctor may generally recommend regulating the pain and fever by using paracetamol instead of aspirin(as it might stimulate bleeding) and increasing fluid ingestion. Children below the age of 12 should not be given aspirin until and unless specially prescribed by the doctor. In severe cases, blood transfusions, intravenous (IV) fluid supplementation and 24 hours hospitalization are required.

Prevention of Dengue Fever

Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET

The patient should take proper bed rest especially during the days when the fever is at its peak and take exclusion from work, school preschool or childcare. People suffering from dengue must stay away from places where they could get bitten by mosquitoes and should stay at home until they are no longer infectious (around 3-5 days).
For avoiding this illness, make sure your surroundings are free of any water logging issues as the Aedes mosquito prefers to breed in stagnant clean water that could be found easily nearby our habitats. Until now, no vaccine has been developed to prevent Dengue virus. The only prevention is to avoid mosquito bites.

  • Cover your skin by wearing long pants, and long-sleeved shirts.
  • Use of mosquito repellents, traps, and nets.
  • Keep all the doors and windows closed especially at dawn, dusk, and early evening to avoid the entry of Dengue mosquitoes.
  • Keep your surrounding clean by removing all the wastes and cleaning the standing water.


Lymphatic Filariasis is also known as ElephantiasisLymphatic Filariasis is also known as Elephantiasis

What is Filariasis?

Filariasis a parasitic disease transmitted by black flies and mosquitoes. These parasites are thin, round, worm-like organisms. They appear white or translucent when observed under a microscope. The life span of filariae would be around 5 to 7 years. During their lifespan, they produce millions of larvae. The size of the female is between 250 and 300 μm long and the size of the males is roughly half as long as the females.

How is Filariasis Transmitted?

The causative organisms for Filariasis is a roundworm of the Filarioidea type. However, this is a vector-borne disease, with the primary vectors being mosquitoes and black flies. The infection spreads when a mosquito bites an already infected individual and then, goes on to bite a healthy person. This causes the larva of this parasite to enter the bloodstream of a healthy host and multiply. The complete stage of the larva to adult filarial worms is carried out within the lymphatic system. Once matured, the adult filarial worm starts to release larval forms called microfilariae. The newly generated microfilariae again enter into the mosquito along with the host’s blood and the cycle repeats itself. This parasitic disease is categorized on the basis of which body part is infected:

  • Lymphatic filariasis: As the name suggests, this type affects the lymphatic system.
  • Subcutaneous filariasis: In this case, the bottom layer of the skin and white part of the eyes are infected by the worms.
  • Serous cavity filariasis: In the case of this disorder, the serous cavity of the abdomen is infected.

There are more than a hundred species of filarial worms currently discovered. Among them, only 8 to 9 are categorized as filarial parasites, which causes infections in human beings.
Filariasis Life Cycle: From larvae to adult Filariasis Life Cycle: From larvae to adult 

Filariasis Symptoms

The general filariasis symptoms during its early stages include:

  • Fever
  • Chills
  • Headache
  • Skin lesions are observed in the beginning stage i.e., between three months to nine months after the insect bite

The filariasis symptoms seen in the later stage include:

  • Blockage in the lymphatic system which leads to oedema
  • Swelling, redness, and pain in the arms and legs
  • Accumulation of pus in cells

The formation of pus in a cell due to the dying worms or a secondary bacterial infection results in:

  • Skin rashes.
  • Abdominal pain
  • Damage to the cornea, choroid, retina and optic nerve finally resulting in loss of vision.
  • The hyper or hypopigmented skin on the face, arms, feet, and other parts of the body.
  • If these symptoms are neglected, the final stage of this disease would result in gross enlargement of the limbs and genitalia in a condition called elephantiasis.

Filariasis Diagnosis

It is quite difficult to diagnose this disease as the symptoms vary from one type to another type and the symptoms diagnosed in the early stage are similar to normal viral and bacterial infections. However, there are a few diagnostic processes carried out through body analysis, physical examination, body fluid test, including a blood test and urine test. Apart from these tests, few blood tests are carried out during the nighttime as the larvae are found most active at night.
A popular diagnostic procedure involves blood being drawn from a larger vein and smeared on a glass slide. This slide is then examined under an electron microscope to check for the presence of parasitic larval roundworms. The presence of Filariasis is examined with the help of the Giemsa stain. The other diagnosis for subcutaneous filariasis is performed with a skin snip test.

Treatment for Filariasis

Currently, there is no vaccine available for filariasis. Scientists are still working on developing a cure for filariasis. Prevention is better than cure. As we all know, this disease is caused by the bite of mosquitoes, it is better to prevent the cause of this disease by:

  • Wearing long sleeves and pants to prevent the bite of mosquitoes
  • Apply DEET- based insect repellents
  • Preventing the build-up of stagnant water
  • Using mosquito mats, coils and nets if possible
  • Though scientifically unproven, dark-coloured clothing attracts mosquitoes, so it might be beneficial to avoid them.

In spite of all these measures, if filariasis is contracted, the best treatment would be a course of anthelmintic drugs or antibiotics. Anthelmintic would be the best option as it directly kills the worms. But if this isn’t feasible, antibiotics are a great alternative because they target the symbiotic bacteria that are present inside the worms. When the antibiotics kill these bacteria, the worms cannot survive and thrive inside their hosts, eventually hampering reproduction.


The parasitic roundworms belong to the family of Filarioidea and Phylum Nematoda. This particular species is known by the name filarial worms or filariae and the infection caused by them is called filariasis. General symptoms of filariasis include fever, chills, headache and skin lesions. Further progression of the disease will disfigure the patient with excessive swelling of the limbs.

The document Common Diseases in Humans - 1 Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET
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