An infectious disease is caused by pathogenic microorganisms entering the body and reproducing, potentially leading to infection and disease. Microbes cause disease via production of toxins, direct invasion, or triggering immune responses. Infections progress through incubation, prodromal, clinical, and decline phases. Diseases are classified as acute, chronic, or latent based on duration and as local or systemic based on location. Primary infections occur in healthy individuals, while secondary infections impact weakened individuals already suffering from a primary infection. Infectious diseases spread through air, water, body fluids, contaminated objects, and vectors.

2.  A disease or disorder caused by infectious agent such as bacteria,
viruses, protozoans of fungus – called infectious disease
 It occurs when an pathogen enters the body and begins to reproduce,
may lead to disease – called infection
 Most of the infectious agent that cause disease are microscopic in size
and these are called microbes
 Mode of action
 Production of poison – ie toxin, enzymes, protein derivatives
 Direct invasion
 Triggering

3. Phases of infectious disease
 Incubation period
The time between infection and appearance of sign and symptoms
 Prodromal phase
Mild, non specific symptoms, that signal onset of some disease
Ex: Cough, sneeze, mild irritation, rashes, pimples etc..,
 Clinical phase
A person experiences typical signs and symptoms of disease
Ex: Fever, cold, inflammation, diarrhea, severe pain etc..,
 Decline phase
 Subsidence of symptoms
 Recovery phase
 Symptoms disappeared, tissues heals, body regain strength

4. Classification of Infectious disease
I) By Duration
Acute: signs and symptoms formed quickly, persist for short time,
may be week to months
ex: cold, fever, swelling
Chronic: Slow start, persist for long time, may be one year
ex: diabetes, heart diseases, cancer etc.,
Latent: A latent infection is an infection that is hidden, inactive, or dormant.
As opposed to active infections, where a virus or bacterium is
actively replicating and potentially causing symptoms
ex: Chicken pox, HIV, hepatitis,

5. 2. By location
Local: infection occurs in the specific area of body
ex: redness eye, tooth ache, swelling in hand, papules on face
Systemic: Systemic means affecting the entire body, rather than a
single organ or body part.
ex: high blood pressure, flu, corona, HIV, many fungal infection.
3. By timing
Primary: Initial infection in a healthy persons
Ex: boils, impetigo etc.,
Secondary: It occurs in a weakened persons by a primary infection
Ex: pneumonia, typhoid, AIDS

6. Examples of infectious disease
 bacterial infection
 viral infection
 fungal infection
 protozoan infection
Transmission of infectious disease
 Air
 Water
 Body fluids
 Contaminated objects
 Vectors etc.,

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