Infection is the process in which a parasite lodges and multiplies in a host body. However, not all infections result in disease. Understanding the types, sources, and transmission of infections is essential in preventing their spread and managing their impact on human health.
Types of Infections
Infections can be classified into several types based on their nature and how they develop in the host.
1. Primary Infection
The first-time infection caused by a parasite in a host is called a primary infection.
2. Reinfection
When the same parasite causes an infection again in the same host, it is called reinfection.
3. Secondary Infection
When a new infection occurs in a host whose immune system is already weakened by a preexisting disease, it is called a secondary infection.
4. Cross Infection
If a person suffering from one infection acquires another infection from an external source, it is termed cross infection.
5. Nosocomial Infection
A nosocomial infection (hospital-acquired infection) is a cross-infection acquired in a hospital setting.
6. Iatrogenic Infection
An iatrogenic infection is caused by medical procedures or drug treatments.
7. Subclinical Infection
When an infection occurs without causing visible symptoms, it is known as a subclinical infection.
8. Latent Infection
In some cases, parasites remain hidden in the body and cause disease when the host’s immunity weakens. This is called a latent infection.
9. Atypical Infection
An atypical infection does not present the usual clinical symptoms of the disease.
Transmission of Infections
The spread of infections depends on three main factors:
- Reservoir – Where the pathogen normally lives and multiplies.
- Mode of Transmission – How the pathogen spreads from the source to a new host.
- Susceptible Host – A person who can become infected.
1. Reservoirs and Sources of Infection
A reservoir is where a pathogen naturally exists and multiplies, while the source is from where it directly infects humans.
Endogenous Sources (Internal Sources)
Pathogens may originate from the body’s normal flora but cause infections when they move to other body parts. Examples include:
- Escherichia coli (normally in intestines) can cause urinary tract infections.
- Viridans streptococci (normally in the mouth) can cause endocarditis.
Exogenous Sources (External Sources)
Most infections originate from external sources, including:
(a) Humans (Cases and Carriers)
- Healthy Carrier – A person who harbors the pathogen without ever showing symptoms.
- Convalescent Carrier – A person who recovered from the disease but still carries the pathogen.
- Temporary Carrier – The pathogen remains in the body for less than six months.
- Chronic Carrier – The pathogen persists for years or even a lifetime.
- Paradoxical Carrier – A carrier infected by another carrier.
- Contact Carrier – A person who acquires the pathogen from an infected individual.
Carriers play a significant role in disease transmission.
(b) Animals
Many infections spread from animals to humans (zoonotic infections), such as:
- Bacterial: Salmonella food poisoning, tuberculosis.
- Viral: Rabies from dogs.
- Protozoal: Leishmaniasis.
- Helminthic: Hydatid disease from dogs.
- Fungal: Dermatophytes from cats and dogs.
(c) Insects (Vectors)
Blood-sucking insects can spread infections, known as arthropod-borne diseases. Vectors can be:
- Mechanical Vectors – Carry pathogens passively, like flies spreading typhoid.
- Biological Vectors – Pathogens multiply inside them, like mosquitoes spreading malaria.
(d) Soil and Water
- Clostridium tetani (tetanus) and Clostridium perfringens (gas gangrene) survive in soil for decades.
- Vibrio cholerae (cholera) and hepatitis viruses spread through contaminated water.
(e) Food
Contaminated food causes diseases like food poisoning, diarrhea, and dysentery.
2. Modes of Transmission
Pathogens spread through various mechanisms:
(i) Contact Transmission
- Direct Contact: STDs like gonorrhea and herpes spread through sexual contact.
- Indirect Contact: Pathogens spread through contaminated objects (fomites) like clothes and towels.
A contagious disease spreads by direct contact, while an infectious disease spreads through various modes.
(ii) Inhalation
Respiratory infections like influenza and tuberculosis spread through airborne droplets released by sneezing, coughing, or speaking.
(iii) Ingestion
Contaminated food and water cause diseases like cholera and food poisoning.
(iv) Inoculation
Some pathogens enter the body directly, such as:
- Rabies virus – through animal bites.
- Tetanus spores – through deep wounds.
- HIV and Hepatitis B – through unsterile syringes or blood transfusions.
(v) Vector-Borne Transmission
- Mosquitoes transmit malaria and dengue.
- Ticks transmit Lyme disease.
- Fleas transmit plague.
(vi) Transplacental Transmission
Some infections cross the placenta from mother to fetus, causing congenital infections, such as:
- Syphilis.
- Rubella virus (can cause congenital malformations).
- Cytomegalovirus and herpes virus infections.
(vii) Iatrogenic and Laboratory Infections
- Iatrogenic infections occur due to medical procedures like injections, surgeries, or blood transfusions.
- Laboratory infections can affect healthcare personnel handling infectious materials.
3. Susceptible Host
A host becomes infected when a pathogen enters through specific portals of entry, including:
- Oral route – Contaminated food and water.
- Respiratory tract – Airborne pathogens.
- Genitourinary tract – STDs.
- Conjunctiva – Eye infections.
- Skin wounds – Tetanus, rabies.
Incubation Period
The time between pathogen entry and the onset of symptoms is called the incubation period.
The severity of an infection depends on:
- Microbial Factors – Pathogen virulence and infectious dose.
- Host Factors – Immunity, age, and underlying conditions.
Conclusion
Understanding infections, their sources, and transmission methods helps prevent their spread and improve public health. Proper hygiene, vaccination, vector control, and food safety are essential in controlling infections.
Source: https://byjus.com/, Microbiology by CP Baveja, Biology Ease