Human Physiology

Pathogens and Parasites: Malaria, Ascariasis, Typhoid, Pneumonia, Common Cold, Amoebiasis, Ringworm, Dengue and Chikungunya

Notes for NEET UG

Introduction

Human beings are constantly exposed to a variety of disease-causing organisms called pathogens. These include viruses, bacteria, protozoa, fungi, helminths (worms), and ectoparasites.

Pathogen

A pathogen is any organism capable of causing disease in a host.

Parasite

A parasite is an organism that lives on or inside another organism (host) and derives nourishment at the host's expense.

Types of Human Pathogens

Group	Examples
Viruses	Dengue virus, Rhinovirus, Influenza virus
Bacteria	Salmonella typhi, Streptococcus pneumoniae
Protozoa	Plasmodium, Entamoeba histolytica
Fungi	Trichophyton, Microsporum
Helminths	Ascaris lumbricoides

1. MALARIA

Definition

Malaria is a mosquito-borne protozoan disease caused by species of Plasmodium.

Causative Organism

Plasmodium vivax
Plasmodium falciparum
Plasmodium malariae
Plasmodium ovale
Plasmodium knowlesi

Most Dangerous Species

Plasmodium falciparum

Causes:

Cerebral malaria
Severe anemia
Multi-organ failure

Vector

Female Anopheles mosquito. Only female mosquitoes transmit malaria because they require blood meals for egg production.

Life Cycle of Plasmodium

Two Hosts

Host	Role
Human	Intermediate host
Female Anopheles	Definitive host

Infective Stage for Humans

Sporozoite

Injected during mosquito bite.

Human Phase

Liver Stage

Sporozoites enter bloodstream.
Reach liver cells.
Multiply asexually.
Produce merozoites.

RBC Stage

Merozoites enter RBCs.
Multiply repeatedly.
RBC ruptures.
More merozoites released.

Malarial Paroxysm

Symptoms occur due to rupture of RBCs and release of toxic substances.

Typical Pattern

Chills
High fever
Sweating

Occurs every:

48 hours (tertian malaria)
72 hours (quartan malaria)

Symptoms

Intermittent fever
Chills
Headache
Body pain
Anemia
Splenomegaly

Severe falciparum malaria:

Cerebral malaria
Coma
Death

Diagnosis

Peripheral blood smear
Rapid antigen tests

Prevention

Mosquito nets
Insecticides
Elimination of stagnant water
Protective clothing

Facts

✓ Female Anopheles mosquito is vector.

✓ Plasmodium enters human body as sporozoites.

✓ Rupture of RBCs causes fever.

2. ASCARIASIS

Definition

Ascariasis is a helminthic disease caused by the roundworm Ascaris lumbricoides.

Transmission

Fecal-oral route through:

Contaminated food
Contaminated water
Unwashed vegetables

Habitat

Adult worms live in: small intestine

Life Cycle

Infective Stage

Embryonated egg

Migration Pathway

Intestine
→ Blood
→ Liver
→ Heart
→ Lungs
→ Trachea
→ Swallowed
→ Intestine

This is called heart-lung migration.

Symptoms

Mild Infection

Abdominal pain
Nausea
Weakness

Heavy Infection

Intestinal blockage
Malnutrition
Growth retardation
Appendicitis

Diagnosis

Stool examination

Prevention

Proper sanitation
Hand washing
Safe drinking water
Washing vegetables

Points

✓ Infection occurs by ingesting Ascaris eggs.

✓ Adult worms live in intestine.

✓ Causes internal bleeding and muscular pain.

3. TYPHOID

Definition

Typhoid is an acute bacterial disease caused by Salmonella typhi.

Transmission

Fecal-oral route

Spread through:

Contaminated food
Contaminated water
Poor hygiene

Incubation Period

7–14 days

Symptoms

Early Symptoms

Sustained high fever
Weakness
Headache

Advanced Symptoms

Abdominal pain
Constipation or diarrhea
Enlarged spleen
Intestinal ulceration

Diagnosis

Widal Test: Detects antibodies against Salmonella.

Prevention

Safe drinking water
Proper sewage disposal
Food hygiene
Vaccination

Carrier State

Recovered individuals may continue to spread bacteria. Classic example:
"Typhoid Mary" Mary Mallon

4. PNEUMONIA

Definition

Pneumonia is a bacterial or viral infection of lung alveoli.

Causative Agents

Bacteria

Streptococcus pneumoniae
Haemophilus influenzae

Viruses

Influenza viruses
Respiratory viruses

Pathology

Alveoli become filled with:

Fluid
Pus

Gas exchange becomes difficult.

Symptoms

Fever
Chills
Cough
Chest pain
Difficulty breathing

Severe Cases

Cyanosis
Respiratory failure

Transmission

Droplet infection

Spread by:

Sneezing
Coughing
Shared utensils

Prevention

Vaccination
Respiratory hygiene
Good nutrition

Point

✓ Alveoli fill with fluid causing breathing difficulty.

5. COMMON COLD

Definition

A viral infection affecting the upper respiratory tract.

Causative Agent

Mainly: Rhinoviruses

Transmission

Airborne droplets
Contaminated surfaces
Direct contact

Symptoms

Runny nose
Sneezing
Nasal congestion
Sore throat
Mild fever

Important Fact

Common cold does NOT affect: Lungs, Alveoli unlike pneumonia.

Prevention

Hand hygiene
Mask use
Avoid touching face

6. AMOEBIASIS (AMOEBIC DYSENTERY)

Definition

A protozoan disease caused by Entamoeba histolytica.

Habitat

Large intestine

Transmission

Fecal-oral route

Through:

Contaminated food
Contaminated water

Reservoir

Humans carrying cysts.

Symptoms

Intestinal Symptoms

Abdominal pain
Constipation
Diarrhea

Characteristic Stool

Contains:

Mucus
Blood

Hence called amoebic dysentery.

Complications

Liver abscess
Intestinal ulceration

Prevention

Sanitation
Safe drinking water
Personal hygiene

Fact

Houseflies act as mechanical carriers.

7. RINGWORM

Definition

Ringworm is a fungal infection of skin, hair or nails. Despite the name, no worm is involved.

Causative Fungi

Microsporum
Trichophyton
Epidermophyton

Trichophyton

Site of Infection

Skin
Nails
Scalp

Symptoms

Circular lesions
Intense itching
Redness
Scaling

Favoring Conditions

Heat
Moisture
Poor hygiene

Common in:

Skin folds
Between toes

Transmission

Direct contact
Shared towels
Shared clothes

Prevention

Dry skin properly
Avoid sharing personal items
Maintain hygiene

8. DENGUE FEVER

Definition

A mosquito-borne viral disease caused by Dengue virus.

Vector

Female Aedes aegypti mosquito

Characteristics of Vector

Day-biting mosquito
Breeds in clean stagnant water

Incubation Period

4–10 days

Symptoms

Classical Dengue

Sudden high fever
Severe headache
Retro-orbital pain
Muscle pain

Nickname: Breakbone fever

Laboratory Findings

Thrombocytopenia
Leukopenia

Severe Dengue

Dengue Hemorrhagic Fever

Bleeding
Plasma leakage
Shock

Dengue Shock Syndrome

Life-threatening condition.

Prevention

Eliminate breeding sites
Mosquito nets
Larval control

9. CHIKUNGUNYA

Definition

A viral disease caused by Chikungunya virus.

Vector

Aedes aegypti

Aedes albopictus

Symptoms

Major Features

High fever
Severe joint pain
Rash
Headache

Distinguishing Feature

Persistent joint pain may continue for: Weeks, Months, sometimes years.

Complications

Usually, non-fatal but highly debilitating.

Prevention

Same as dengue:

Avoid mosquito bites
Destroy breeding sites

COMPARISON TABLE FOR NEET

Disease	Pathogen	Type	Vector/Transmission
Malaria	Plasmodium	Protozoan	Female Anopheles
Ascariasis	Ascaris lumbricoides	Helminth	Contaminated food/water
Typhoid	Salmonella typhi	Bacterium	Fecal-oral
Pneumonia	Streptococcus pneumoniae	Bacterium	Droplet infection
Common Cold	Rhinovirus	Virus	Airborne droplets
Amoebiasis	Entamoeba histolytica	Protozoan	Contaminated food/water
Ringworm	Trichophyton/Microsporum	Fungus	Direct contact
Dengue	Dengue virus	Virus	Aedes aegypti
Chikungunya	Chikungunya virus	Virus	Aedes mosquitoes

HIGH-YIELD NEET ONE-LINERS

Malaria is caused by Plasmodium and transmitted by female Anopheles mosquito.
Infective stage of Plasmodium for humans is sporozoite.
Ascariasis is caused by Ascaris lumbricoides.
Typhoid is caused by Salmonella typhi.
Widal test is used for diagnosis of typhoid.
Pneumonia affects alveoli of lungs.
Common cold is mainly caused by rhinoviruses.
Amoebiasis is caused by Entamoeba histolytica.
Houseflies act as mechanical carriers of amoebiasis.
Ringworm is caused by dermatophyte fungi.
Dengue is transmitted by Aedes aegypti mosquito.
Chikungunya is characterized by severe persistent joint pain.
Dengue is also known as breakbone fever.
Plasmodium falciparum causes the most severe malaria.
Ringworm is a fungal disease and not caused by a worm.

NEET RAPID REVISION TABLE

Disease	Causative Agent	Key Symptom	Diagnostic Clue
Malaria	Plasmodium	Periodic fever	Blood smear
Ascariasis	Ascaris	Intestinal blockage	Stool exam
Typhoid	Salmonella typhi	Sustained fever	Widal test
Pneumonia	S. pneumoniae	Breathing difficulty	Chest findings
Common Cold	Rhinovirus	Sneezing	Clinical diagnosis
Amoebiasis	Entamoeba histolytica	Bloody mucus stool	Stool examination
Ringworm	Trichophyton	Circular itchy lesions	Fungal examination
Dengue	Dengue virus	Breakbone fever	NS1/serology
Chikungunya	Chikungunya virus	Severe joint pain	Serology/PCR

Must Memorize for NEET

Protozoan Diseases: Malaria, Amoebiasis
Helminthic Disease: Ascariasis
Bacterial Diseases: Typhoid, Pneumonia
Viral Diseases: Common Cold, Dengue, Chikungunya
Fungal Disease: Ringworm

Tuesday, June 9, 2026

Pathogens and Parasites: Malaria, Ascariasis, Typhoid, Pneumonia, Common Cold, Amoebiasis, Ringworm, Dengue and Chikungunya