Bacterial Diseases (Typhoid, Pneumonia)

NCERT grounding

NCERT Class 12, Chapter 7 (Section 7.1 — Common Diseases in Humans) anchors this subtopic. The textbook opens the disease catalogue with two bacterial illnesses, then proceeds to viruses, protozoans, helminths and fungi. The bacterial entries are short but content-dense: every clause about pathogen genus, mode of transmission, symptom set and diagnostic test is regularly turned into a one-mark match-the-following question.

The exact NCERT wording for typhoid begins: "Salmonella typhi is a pathogenic bacterium which causes typhoid fever in human beings." For pneumonia it states: "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." NIOS Biology Chapter 29 supplements these with brief notes on cholera (Vibrio cholerae), diphtheria (Corynebacterium diphtheriae) and leprosy.

Typhoid, pneumonia and the bacterial cluster

Bacterial pathogens are prokaryotic, unicellular organisms that breach a host's barriers, multiply, and damage tissue either by direct invasion or by toxins. In Chapter 7, NCERT introduces only the bacteria that cause typhoid and pneumonia in detail, mentioning dysentery, plague and diphtheria by name as further examples. The chapter elsewhere notes that polio, diphtheria, pneumonia and tetanus have been controlled to a large extent by the use of vaccines — a sentence that turns up routinely in single-statement objective questions.

For NEET preparation, bacterial diseases sit alongside viral, protozoan, helminthic and fungal diseases in Section 7.1. Examiners almost always frame the question as a four-row matching exercise: disease → pathogen → vector/test → symptom. Knowing the precise binomial of Salmonella typhi and that both Streptococcus pneumoniae and Haemophilus influenzae cause pneumonia is the difference between two safe marks and two careless drops.

A bacterium qualifies as a pathogen when it satisfies three operational requirements: it can survive the entry route (low gastric pH for enteric pathogens such as Salmonella; aerosol shear for respiratory pathogens such as Streptococcus); it multiplies inside the host using host nutrients; and it produces tissue damage either directly through invasion or indirectly through toxins. Salmonella typhi achieves all three by surviving stomach acid, invading intestinal lymphoid tissue and disseminating through blood. Streptococcus pneumoniae colonises the upper respiratory tract and reaches the alveoli when host defences (mucociliary escalator, cough reflex, alveolar macrophages) are weakened — for example, after a viral cold. This sets up the classical "secondary bacterial pneumonia" pattern that paediatricians watch for.

Typhoid — Salmonella typhi

Typhoid fever, the first bacterial disease NCERT introduces, is caused by Salmonella typhi, a Gram-negative rod-shaped bacillus. The pathogen enters the small intestine through food and water contaminated with it, and from there migrates through blood to other organs. This intestinal–haematogenous route is the reason intestinal perforation is the most feared complication.

The textbook lists five symptoms in a fixed order: sustained high fever (39°–40°C), weakness, stomach pain, constipation, headache, with loss of appetite added. In severe cases intestinal perforation and death may occur. Diagnosis is by the Widal test, a serological agglutination assay that detects antibodies against O and H antigens of Salmonella in the patient's serum.

NCERT also recounts "a classic case in medicine, that of Mary Mallon nicknamed Typhoid Mary". Mallon was a cook by profession and an asymptomatic typhoid carrier who continued to spread typhoid for several years through the food she prepared. The anecdote illustrates a central epidemiological idea — a person can carry and shed Salmonella typhi without ever appearing ill, which is why public hygiene measures (clean drinking water, periodic disinfection of reservoirs, hygiene in catering) form NCERT's standard prevention list for food-and-water borne diseases such as typhoid, amoebiasis and ascariasis.

Pneumonia — Streptococcus and Haemophilus

Pneumonia is the second bacterial disease NCERT details. The textbook names two bacteria together: "Bacteria like Streptococcus pneumoniae and Haemophilus influenzae are responsible for the disease pneumonia in humans." They infect the alveoli — the 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 symptom set NCERT prescribes is: fever, chills, cough and headache. In severe cases, the lips and finger nails may turn gray to bluish in colour — this reflects systemic hypoxaemia (cyanosis) caused by impaired gas exchange across fluid-filled alveoli. Transmission is droplet-mediated: 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.

Diphtheria, tetanus, cholera

NCERT names dysentery, plague and diphtheria as additional bacterial diseases without giving detail; the chapter later highlights that polio, diphtheria, pneumonia and tetanus have been controlled to a large extent by the use of vaccines. NIOS Biology Chapter 29 fills in the brief profiles below. NEET has occasionally tested the causative agent of cholera and diphtheria, so the binomials are worth memorising even though Class 12 NCERT does not elaborate on them.

Diphtheria, caused by the rod-shaped bacterium Corynebacterium diphtheriae, is a droplet-borne infection of children between one and five years of age. NIOS gives an incubation period of two to four days. The defining symptom is the deposition of a tough greyish membrane in the throat that can clog the airway; the bacterium itself secretes a potent exotoxin that drives tissue damage. Prevention rests on the DPT vaccine — the "D" stands for diphtheria toxoid — which is one of the cornerstone immunisations in India's universal immunisation programme.

Tetanus, caused by Clostridium tetani, is fundamentally different in mode of entry: the spores live in soil and enter the body through deep puncture wounds, not through air or food. The bacterium then releases a neurotoxin (tetanospasmin) that produces sustained, painful muscle spasms — historically called "lockjaw" because the jaw muscles seize first. The "T" of DPT is tetanus toxoid. Cholera, the third disease in this NCERT-by-name cluster, is caused by the comma-shaped Vibrio cholerae and is transmitted by contaminated food and water with the housefly acting as a mechanical carrier. Its hallmark is acute watery diarrhoea leading to severe dehydration; oral rehydration solution (ORS) is the standard intervention.

A note on common cold (context only)

Although the dispatch lists common cold inside this bacterial cluster for chapter context, NCERT is explicit: "Rhino viruses represent one such group of viruses which cause one of the most infectious human ailments — the common cold." Common cold is therefore viral, not bacterial. We mention it here only because it is taught in the same sequence in Chapter 7 — full treatment lives in the sibling Viral Diseases (Common Cold) page. Examiners frequently exploit this confusion in match-the-following questions (e.g. NEET 2024 Q.177 paired "Common cold — Rhinoviruses").

Worked examples

Common confusion & NEET traps