Common Diseases in Human



  • Pathogens and Disease:

- Bacteria, viruses, fungi, protozoans, helminths can cause diseases. 

- These disease-causing organisms are termed pathogens. 

  • Parasites as Pathogens:

- Most parasites are pathogens, harming the host by residing in or on them. 

- Pathogens enter the body through various means, multiplying and disrupting vital activities. 

  • Adaptation of Pathogens:

- Pathogens must adapt to the host environment. 

- Example: Gut pathogens adapt to survive in the stomach's low pH and resist digestive enzymes. 

  • Understanding the diversity of pathogens, their modes of entry, and adaptive strategies is crucial for devising effective preventive and therapeutic measures in the field of medicine.


Bacterial Diseases

Typhoid Fever:

  • Salmonella typhi is a bacterium causing typhoid fever in humans. 
  • Mode of Entry:

   Typically enters the small intestine through contaminated food and water. 

  • Spread within the Body: Migrates to other organs through the bloodstream. 
  • Symptoms of Typhoid Fever: Sustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache, and loss of appetite. 
  • Severity and Complications: Severe cases may lead to intestinal perforation and, in extreme cases, death. 
  • Diagnostic Confirmation: Typhoid fever can be confirmed through the Widal test. 
  • Case Example: Typhoid Mary:

- Notable case in medical history. 

- Mary Mallon (Typhoid Mary), a cook, was a carrier of typhoid and unknowingly spread the disease through the food she prepared. 

- Emphasizes the importance of identifying carriers to prevent the spread of infectious diseases. 


  • Streptococcus pneumoniae and Haemophilus influenzae are bacteria causing pneumonia in humans. 
  • Affected Area: Pneumonia infects the alveoli, which are air-filled sacs in the lungs. 
  • Consequences of Infection: Infection leads to the filling of alveoli with fluid, causing severe respiratory issues. 
  • Symptoms: Common symptoms include fever, chills, cough, and headache. 
  • Severe Cases: In severe pneumonia, lips and fingernails may turn gray to bluish in color. 
  • Transmission:

- Healthy individuals can acquire the infection by inhaling droplets/aerosols released by an infected person. 

  • Sharing glasses and utensils with an infected person also poses a risk. 



  • Other Bacterial Diseases:

1. Dysentery: 

Bacterial infection causes inflammation of the intestines, leading to diarrhea with blood. 

2. Plague:

- Bacterial disease caused by Yersinia pestis, often transmitted through fleas. 

- The historical significance of the plague, particularly the Black Death in the 14th century, lies in its profound impact on population, society, and culture. 

- It led to a significant reduction in population, altering social structures, inspiring artistic expression, prompting early public health measures, influencing religious thought, and shaping urban planning practices. 

- The Black Death's consequences extended across demographics, economies, and various aspects of human life, leaving a lasting imprint on history.


Bacterial infection affecting the respiratory system, causing a thick coating in the throat, leading to breathing difficulties. 


Viral Diseases

Common Cold:

  •  Rhino Viruses:

A group of viruses causing one of the most infectious human ailments – the common cold. 

  • Targeted Areas:

Infect the nose and respiratory passages, distinguishing them from lung infections. 

  • Common Cold Characteristics:

- Characterized by nasal congestion, discharge, sore throat, hoarseness, cough, headache, and tiredness. 

- Typically lasts for 3-7 days. 

  • Transmission:

- Spread through droplets produced by coughs or sneezes of an infected person. 

- Transmission also occurs via contaminated objects like pens, books, cups, doorknobs, computer keyboards, or mouse. 

  • Awareness of transmission through respiratory droplets and contaminated objects emphasizes the importance of hygiene to prevent the spread of viral infections.


Protozoan Diseases


  • Causative Agent: Protozoan parasite named Plasmodium sps. 
  •  Malaria Overview:Disease fought for many years, predominantly transmitted by infected female Anopheles mosquitoes. 
  • Plasmodium Species: Different types of malaria caused by various Plasmodium species: P. vivax, P. malaria, and P. falciparum.
  • Severity Levels: Malignant malaria, caused by P. falciparum, is the most serious and potentially fatal. 
  • Life Cycle of Plasmodium:

Entry into Human Body:

- Sporozoites (infectious form) enter the human body through the bite of an infected female Anopheles mosquito. 

Multiplication and Rupture:

- Parasites multiply in liver cells and later attack red blood cells (RBCs), causing their rupture. 

Symptoms and Fever:

- Rupture releases a toxic substance, haemozoin, leading to recurring chills and high fever every three to four days.

Transmission to Mosquito:

- When a female Anopheles mosquito bites an infected person, parasites enter and undergo further development. 

Completing the Life Cycle:

- Parasites multiply within the mosquito, forming sporozoites stored in salivary glands. 

- Subsequent bites transmit sporozoites to humans, completing the cycle. 

  • Two-Host Requirement:

- Malarial parasite requires two hosts – human and mosquitoes – for its complete life cycle. 

- Female Anopheles mosquito serves as the vector, transmitting the parasite.






Amoebiasis (Amoebic Dysentery)

  • Causative Protozoan: Entamoeba histolytica, a protozoan parasite residing in the large intestine of humans. 
  • Disease Overview: Causes amoebiasis, commonly known as amoebic dysentery. 
  • Symptoms: Manifestations include constipation, abdominal pain, cramps, and stools with excessive mucus and blood clots. 
  • Transmission Dynamics:

- Mechanical Carriers:

Houseflies serve as mechanical carriers, transmitting the parasite from the feces of an infected person to food and food products. 

- Contamination Source:

Main sources of infection are drinking water and food contaminated by fecal matter. 

Transmission Route:

Houseflies contaminate food by transferring the parasite from infected feces, leading to subsequent ingestion by humans.


Helminthic Diseases


  • Causative Agent: Parasite: Ascaris lumbricoides, a common intestinal roundworm. 
  • Symptoms: Internal bleeding, muscular pain, fever, Anaemia., blockage of the intestinal passage. 
  • Mode of Transmission:

- Egg Dissemination:

Parasite eggs are excreted with the feces of infected individuals. 

- Contamination Sources:

Contaminated soil, water, plants, etc. 

- Acquisition by Healthy Individuals:

Through ingestion of contaminated water, vegetables, fruits, etc. 

  • Health Impact: Causes a range of symptoms affecting the gastrointestinal system and overall well-being. 
  •  Prevention and Control:

- Hygiene Practices:

Proper sanitation to minimize fecal contamination.

Thorough washing of vegetables and fruits.

Access to clean drinking water. 

- Health Education:

Raise awareness about the importance of hygiene in preventing Ascariasis.



Elephantiasis or Filariasis

  • Causative Agent: Parasite: Wuchereria bancrofti and Wuchereria malayi, filarial worms. 
  •  Characteristics: Slowly developing chronic inflammation of organs, commonly lymphatic vessels in lower limbs. 
  • Symptoms:

- Chronic Inflammation:

Persistent swelling due to lymphatic system disruption. 

- Gross Deformities:

Particularly in the genital organs. 

  • Transmission: Female mosquito vectors (e.g., Culex quinquefasciatus) transmit the pathogens during a blood meal. 
  •  Geographical Distribution: Commonly prevalent in tropical and subtropical regions. 
  •  Health Impact: Causes long-term disability and disfigurement.  
  • Preventive Measures:

- Vector Control:

Mosquito control programs to reduce transmission. 

- Personal Protection:

Use of bed nets and protective clothing. 

- Mass Drug Administration (MDA):

Distribution of antifilarial drugs in endemic areas. 

  • Public Awareness: Health education campaigns emphasizing preventive measures and early treatment.



Fungal Diseases


  • Causative Fungi: Genera Microsporum, Trichophyton, and Epidermophyton. 
  • Symptoms:

- Appearance of Lesions:

Dry, scaly lesions on various body parts (skin, nails, scalp).

- Accompanying Itching:

Intense itching associated with the lesions. 

  • Fungi Growth Conditions:

- Favorable Environment:

Thrive in heat and moisture. 

- Prevalent Areas:

Skin folds like those in the groin or between toes. 

  • Mode of Transmission:

- Generally acquired from soil. 

- Transmission through towels, clothes, or combs used by infected individuals. 

  • Preventive Measures:

- Regular cleaning and drying of skin folds. 

- Avoid sharing personal items with infected individuals. 

  • Treatment: Topical or oral antifungal medications for effective treatment. 
  • Public Awareness: Health education campaigns on maintaining hygiene and recognizing early symptoms.



Prevention and Control of Infectious Diseases

Disease Prevention

  • Personal Hygiene Measures:

- Body Cleanliness:

Regular personal cleanliness to prevent the spread of infectious agents. 

- Safe Consumables:

Consumption of clean drinking water, uncontaminated food, vegetables, and fruits. 

  • Public Hygiene Measures:

- Waste Disposal:

Proper disposal of waste and excreta to prevent the transmission of diseases. 

- Cleaning and Disinfection:

Periodic cleaning and disinfection of water reservoirs, pools, cesspools, and tanks. 

- Hygiene in Public Spaces:

Observance of standard hygiene practices in public catering establishments. 

  •  Disease-Specific Measures:

Food and Waterborne Diseases (e.g., Typhoid, Amoebiasis, Ascariasis):

- Emphasize clean food and water consumption.

- Prevent transmission through contaminated food and water. 

Airborne Diseases (e.g., Pneumonia, Common Cold):

- Avoid close contact with infected persons or their belongings. 

Vector-Borne Diseases (e.g., Malaria, Filariasis):

- Control or eliminate vectors and breeding places.

- Measures include water stagnation prevention, mosquito net use, introduction of mosquito-eating fishes, and insecticide spraying. 

  • Biological Advancements:

Vaccines and Immunization:

- Eradication of deadly diseases like smallpox through vaccination. 

- Control of diseases like polio, diphtheria, pneumonia, and tetanus. 

Biotechnology Contributions:

- Ongoing developments for newer and safer vaccines. 

  •  Drug Discoveries:

- Antibiotics and Treatments:

Effective treatment of infectious diseases through antibiotic and drug discoveries.