Table of Contents
Transmission of Infectious Diseases
Basic Concepts: Infection, Colonization, Contagiousness
Before looking at pathways and the course of infections, it helps to clarify a few key terms:
- Infection: When a pathogen (virus, bacterium, fungus, protist, or animal parasite) enters the body, multiplies, and triggers a reaction from the host. Infection can be symptomatic (with signs of disease) or asymptomatic (no obvious symptoms).
- Colonization: Presence and multiplication of microorganisms on body surfaces (e.g., skin, gut, throat) without harming the host. Many colonizing microbes are harmless or beneficial.
- Contagiousness (infectivity): How easily a pathogen spreads from one host to another. Pathogens differ widely in this property.
- Virulence: The degree of harmfulness or ability to cause severe disease.
An infected person can spread pathogens even if they feel healthy. Such individuals are particularly important for transmission.
Sources and Reservoirs of Infection
A reservoir is the habitat where a pathogen normally lives and multiplies, from which humans can become infected.
Human Reservoirs
- Sick individuals (clinical infections)
People who show symptoms often shed high numbers of pathogens in: - respiratory secretions (cough, sneeze)
- stool, urine
- blood, skin lesions, genital secretions
- Carriers (asymptomatic or post-disease)
Persons who harbor a pathogen and can transmit it, but show no or only mild symptoms: - Incubatory carriers: Already contagious during the incubation period (e.g., influenza).
- Convalescent carriers: Still excrete pathogens after apparent recovery (e.g., some intestinal infections).
- Chronic carriers: Long-term carriers over months or years (classic example: chronic carriers of Salmonella Typhi).
Carriers are harder to identify and control because they appear healthy.
Animal Reservoirs (Zoonotic Reservoirs)
Zoonoses are infectious diseases that are naturally transmissible between vertebrate animals and humans, such as:
- Rabies (reservoirs: wild mammals, dogs)
- Certain influenza strains (birds, pigs)
- Plague (rodents)
- Many parasitic infections (e.g., some worms, protozoa)
Transmission can occur through bites, contact with saliva, excreta, meat, or via vectors (e.g., fleas, ticks, mosquitoes).
Environmental Reservoirs
Some pathogens can persist and multiply outside living hosts, e.g.:
- Water (e.g., cholera bacteria in contaminated water)
- Soil (e.g., spores of tetanus or anthrax bacteria)
- Food (bacterial growth in inadequately cooled or stored food)
These reservoirs are important in outbreaks linked to contamination of drinking water or food chains.
Routes (Pathways) of Transmission
Transmission describes how a pathogen moves from its reservoir to a new host. Often, multiple routes are possible for one pathogen.
1. Direct Contact Transmission
Occurs through immediate physical contact between the source and a susceptible person.
- Skin-to-skin contact
E.g., transmission of certain skin infections, scabies mites. - Mucous membrane contact
Kissing, direct contact of moist surfaces (mouth, genitals, eyes).
Sexual Transmission
A special form of direct contact, involving exchange of genital secretions (semen, vaginal secretions) and often blood:
- Pathogens can infect via:
- micro-injuries in mucous membranes,
- contact with infected fluids.
- Examples (details in other chapters): HIV, many bacteria, viruses, and parasites.
Protective measures include barrier methods (e.g., condoms), screening programs, and reducing the number of contacts.
2. Indirect Contact Transmission (via Objects)
Also called fomite transmission. Pathogens are transferred via contaminated inanimate objects:
- door handles, handrails
- towels, bedding, clothing
- medical instruments, needles
- toys, keyboards, smartphones
Pathogens must survive for some time outside the body to use this route. Many bacteria and some viruses can persist on dry surfaces; others die quickly.
Hand hygiene and surface disinfection are critical to reducing this route of spread, especially in hospitals.
3. Droplet Transmission
Short-range transfer via relatively large, moist droplets produced when:
- coughing
- sneezing
- talking, singing
- certain medical procedures (e.g., suctioning)
These droplets:
- are relatively heavy,
- travel typically less than 1–2 meters,
- fall quickly onto surfaces or directly into the eyes, nose, or mouth of another person.
Droplet transmission explains why:
- keeping distance,
- wearing surgical masks,
- covering coughs and sneezes
can significantly reduce the spread of many respiratory infections.
4. Airborne Transmission (Aerosols)
Here, pathogens are carried in tiny particles or droplet nuclei that:
- remain suspended in the air for minutes to hours,
- can travel longer distances with air currents,
- are inhaled deeply into the respiratory tract.
Airborne transmission is particularly relevant for pathogens adapted to survive in the air. Control measures include:
- good ventilation,
- special air filters in hospitals,
- specific respiratory protective masks (e.g., respirators).
5. Fecal–Oral Transmission
Pathogens are excreted in stool (feces) and reach the mouth of a new host. Typical routes:
- contaminated drinking water
- contaminated food (e.g., through unwashed hands or flies)
- poor hand hygiene after using the toilet
This route is especially important for intestinal pathogens. Prevention relies on:
- safe drinking water,
- wastewater treatment,
- handwashing with soap,
- safe food handling and cooking.
6. Foodborne Transmission
Food can serve as a vehicle:
- Primary contamination: Pathogen already present in the food source (e.g., infected animal).
- Secondary contamination: Occurs during processing, transport, or preparation (e.g., via hands, equipment).
- Multiplication in food: Improper refrigeration or long storage at warm temperatures allows microbes to multiply.
Some pathogens form toxins in food. In such cases, the toxin causes illness even if the bacteria are dead when eaten.
7. Waterborne Transmission
Occurs when drinking or bathing water is contaminated with pathogens:
- often overlaps with fecal-oral transmission,
- may also involve free-living environmental pathogens.
Important preventive measures include:
- water treatment and disinfection,
- protecting water sources from sewage and animal waste,
- boiling water where quality is uncertain.
8. Vector-Borne Transmission
A vector is a living organism—usually an arthropod like a mosquito, tick, or flea—that carries pathogens from one host to another.
Key forms:
- Biological vectors:
- Pathogens multiply or develop inside the vector (e.g., malaria parasites in mosquitoes).
- The vector is often essential for the pathogen’s life cycle.
- Mechanical vectors:
- Pathogens are transported on the body surface (e.g., flies carrying bacteria from feces to food) without development in the vector.
Transmission routes include:
- bites (injected with saliva)
- contact with contaminated feces of the vector on the skin or mucous membranes
Prevention can include:
- vector control (insecticides, removing breeding sites),
- bed nets and protective clothing,
- repellents and vaccination (where available).
9. Bloodborne Transmission
Pathogens in blood can be transmitted by:
- sharing contaminated needles or syringes
- transfusion of contaminated blood or blood products
- medical or cosmetic procedures with insufficiently sterilized instruments
- injuries with sharp objects contaminated with blood
Prevention strategies include:
- sterile disposable equipment,
- screening blood donations,
- careful handling and disposal of sharps,
- safe practices in tattooing and piercing.
10. Vertical Transmission (Mother-to-Child)
From an infected mother to her child:
- During pregnancy (transplacental)
Pathogens cross the placenta and infect the fetus. - During birth
The child is exposed to pathogens in the birth canal. - Through breast milk
Some pathogens can be passed via breastfeeding.
Consequences can range from mild to severe, including miscarriage, developmental disorders, or chronic infections. Prenatal screening and treatment, as well as specific delivery or feeding strategies, reduce risks.
Stages in the Course of an Infectious Disease
Many infectious diseases follow a broadly similar temporal pattern, though duration and expression can vary greatly.
1. Exposure and Entry (Invasion)
Exposure is contact between the host and the pathogen. Invasion is the actual entry into the body.
- Pathogens use specific entry portals:
- respiratory tract (inhalation)
- digestive tract (ingestion)
- skin (wounds, insect bites, injections)
- urogenital tract
- conjunctiva (eye surface)
- Some only infect when they reach a specific tissue (e.g., intestinal lining, lung tissue).
Not every exposure leads to infection; the host’s defenses may prevent entry or eliminate the pathogen early.
2. Incubation Period
The incubation period is the time from infection to the onset of first symptoms.
- Can be hours, days, weeks, or even years.
- Depends on:
- rate of pathogen multiplication,
- distance to target tissue,
- host’s immune response.
During incubation:
- The pathogen may spread within the body.
- Many diseases are already contagious in this phase, even before symptoms arise. This facilitates unnoticed spread.
3. Prodromal Phase
In some diseases, a short phase with nonspecific early symptoms:
- fatigue, malaise
- slight fever
- headache, muscle aches
- mild discomfort in affected organ systems
These symptoms are hard to distinguish from other illnesses but mark the transition from incubation to the main phase of illness.
4. Acute Phase (Clinical Manifestation)
The pathogen is present in high numbers, and characteristic symptoms and signs of the disease appear.
Typical features include:
- Fever and other general inflammatory signs
- Local symptoms depending on organ involvement:
- respiratory: cough, shortness of breath
- gastrointestinal: diarrhea, vomiting
- skin: rashes, blisters, sores
- nervous system: headache, paralysis, confusion, seizures
- The intensity reflects:
- pathogen virulence,
- extent of tissue damage,
- host immune reaction (overreactions can also cause harm).
The acute phase is often when:
- infectiousness is greatest,
- complications may arise (e.g., dehydration, organ failure, secondary bacterial infections).
5. Decline and Convalescence
If the immune system gains the upper hand (or therapy is effective):
- Pathogen numbers decrease.
- Symptoms gradually improve.
- Body repairs damaged tissues.
Convalescence is the recovery period until normal function is largely restored.
Important aspects:
- Persons may still carry and shed pathogens (convalescent carriers).
- Fatigue and reduced performance are common, especially after severe disease.
- Some infections result in chronic damage (e.g., scars, organ impairment) despite apparent recovery.
Alternative Courses: Subclinical, Chronic, and Latent Infections
Not all infections follow the classic clear pattern.
Subclinical (Asymptomatic) Infection
- Infection occurs, the immune system responds, but no noticeable symptoms develop.
- The person may develop immunity (e.g., detectable antibodies).
- The infected person can still spread the pathogen in some cases.
Subclinical infections are significant in population-level spread because they are rarely detected.
Chronic Infection
- The pathogen persists in the host for months or years.
- Symptoms may be:
- constantly present but mild,
- fluctuating (relapsing).
- Transmission to others may remain possible over long periods.
Chronic infections may lead to progressive tissue damage and long-term health consequences.
Latent Infection and Reactivation
- The pathogen remains in the body in a dormant (inactive) state after the initial infection.
- No symptoms and little or no replication during latency.
- Under certain conditions (e.g., weakened immunity, stress), the pathogen can reactivate and cause disease again.
Latency is distinct from chronic active infection; here the pathogen is “hidden” much of the time.
Outcomes of Infection
The interaction between host and pathogen can lead to different endpoints:
- Complete elimination: The immune system clears the pathogen; only immunological memory remains.
- Persistent infection: Chronic or latent infection remains.
- Colonization without disease: The microbe becomes part of the normal flora.
- Death of the host: In severe cases or with highly virulent pathogens.
- Carrier state: The host appears healthy but continues to transmit the pathogen.
Which outcome occurs depends on:
- pathogen characteristics,
- infectious dose,
- entry route,
- host factors.
Host and Environmental Factors Influencing Transmission and Course
The same pathogen can cause very different courses of disease in different people. Important influences include:
Host-Related Factors
- Age
Newborns and the elderly often have higher susceptibility or more severe courses. - Immune status
- immunodeficiencies (e.g., due to HIV, certain drugs, genetic defects),
- previous infections or vaccinations (acquired immunity).
- Nutritional status
Malnutrition, especially protein and micronutrient deficiencies, weakens defense mechanisms. - Chronic diseases
E.g., diabetes, lung or heart disease can complicate infections. - Genetic factors
Some genetic variants increase or decrease susceptibility.
Pathogen-Related Factors
- Infectious dose (how many microbes are needed for infection)
- Virulence factors (toxins, adhesion molecules, immune evasion strategies)
- Ability to survive outside the host (e.g., in air, water, on surfaces)
- Adaptation to specific transmission routes (e.g., survival in stomach acid, resistance to drying)
Environmental and Social Factors
- Population density and crowding
Facilitates droplet, airborne, and contact transmission. - Hygiene, sanitation, and water supply
Critical for controlling fecal–oral and fomite transmission. - Healthcare structures
Hospitals can be places of nosocomial infections if hygiene is insufficient; they also implement measures to interrupt chains of infection. - Behavior and cultural practices
Handwashing, food preparation habits, sexual behavior, vaccination acceptance. - Travel and mobility
Move pathogens quickly between regions and continents.
Chains of Infection and Points of Intervention
The chain of infection summarizes the steps necessary for transmission:
- Reservoir (source of pathogen)
- Exit portal (where pathogen leaves: e.g., respiratory tract, stool, blood)
- Transmission route (direct, indirect, vector, etc.)
- Entry portal (where pathogen enters: e.g., mucous membranes, wounds)
- Susceptible host
Interrupting any link can prevent new infections. Typical measures:
- Treating or isolating sources (e.g., sick individuals, contaminated water)
- Blocking exit and entry portals (masks, condoms, covering wounds)
- Breaking transmission routes (hand hygiene, disinfection, vector control)
- Reducing host susceptibility (vaccination, good nutrition, treatment of underlying diseases)
Understanding transmission pathways and the course of infectious diseases is the basis for designing effective prevention, control, and treatment strategies, which will be explored in more detail in other chapters.