Table of Contents
Infectious diseases are disorders caused by microscopic organisms that invade the human body, multiply, and interfere with normal functions. In this chapter, the focus is on how such diseases appear and spread in humans, what distinguishes them from non‑infectious diseases, and which general patterns they follow. The specific pathogen groups and example diseases (viruses, bacteria, fungi, protists, animals) will be treated in the following subchapters.
What Makes a Disease “Infectious”?
A disease is called infectious if:
- It is caused by living or quasi‑living agents (pathogens), such as:
- Viruses
- Bacteria
- Fungi
- Protists
- Small animals (e.g., worms, mites)
- The pathogen can be transmitted from one host to another (directly or indirectly).
- The pathogen multiplies in or on the host.
In contrast, non‑infectious diseases (e.g., many genetic disorders, injuries, poisoning) are not caused by transmissible pathogens.
Important terms:
- Pathogen: Any organism or particle that can cause disease.
- Host: The organism in which the pathogen lives and reproduces.
- Infection: The successful invasion and multiplication of a pathogen in the host.
- Colonization: Presence and possible multiplication of microbes without causing disease (e.g., many gut bacteria).
- Infectious disease: The combination of infection and resulting symptoms.
Routes of Transmission
Pathogens reach humans via specific routes. Knowing these is essential for understanding prevention and control.
1. Direct Contact
Pathogen transfer occurs through immediate physical contact between people:
- Skin‑to‑skin contact: e.g., shaking hands, hugging.
- Mucosal contact: kissing, sexual contact.
- Body fluids: blood, semen, vaginal secretions, saliva, breast milk.
Many skin, sexually transmitted, and some blood‑borne diseases spread this way.
2. Indirect Contact (via Objects and Surfaces)
Pathogens can survive for some time on inanimate objects (fomites):
- Door handles, railings
- Towels, cutlery, toys
- Medical instruments
If a second person touches a contaminated object and then their mouth, nose, eyes, or a wound, infection can occur.
3. Droplet Infection
When an infected person:
- Coughs
- Sneezes
- Speaks loudly or sings
they release small liquid droplets containing pathogens. These can:
- Land directly on mucous membranes of others (eyes, nose, mouth).
- Fall onto surfaces and later be picked up (combination of droplet and indirect contact).
Droplet transmission usually acts over short distances (typically up to about 1–2 m in everyday situations).
4. Airborne Transmission (Aerosols)
Very small droplets (aerosols) can remain suspended in the air for a long time and disperse through rooms or ventilation systems. Unlike classic droplets, aerosols can:
- Travel greater distances.
- Accumulate in poorly ventilated spaces.
This route is crucial for some highly contagious respiratory infections.
5. Fecal–Oral Route
Pathogens from the intestine are excreted with feces and reach another person’s mouth, for example via:
- Contaminated hands
- Contaminated drinking water
- Contaminated food
- Inadequate sanitation
This route is typical for many gastrointestinal infections.
6. Food‑ and Waterborne Transmission
Here, pathogens multiply in or on:
- Food (e.g., meat, eggs, dairy products, raw vegetables)
- Drinking water and recreational water
Transmission occurs via ingestion. Improper storage, insufficient cooking, and contaminated water sources are key risk factors.
7. Vector‑Borne Transmission
Vectors are living carriers that transmit pathogens between hosts without themselves necessarily becoming ill. Important examples:
- Blood‑sucking insects (e.g., mosquitoes)
- Ticks
- Fleas, lice, other arthropods
The pathogen often develops or multiplies in the vector before being passed to humans, typically via a bite.
8. Vertical Transmission (Mother to Child)
Pathogens can be transferred:
- During pregnancy via the placenta
- During birth via contact with genital secretions or blood
- After birth via breast milk or close contact
Such infections can particularly endanger the developing fetus or newborn.
From Infection to Disease: General Course
The course of an infectious disease often follows a characteristic pattern, though details differ among pathogens and individuals.
1. Exposure and Infection
- Exposure: Contact between host and pathogen.
- Infection: Pathogen overcomes external barriers (skin, mucous membranes) and enters the body, then reaches a suitable tissue and begins to multiply.
Not every exposure leads to infection, and not every infection leads to illness; the immune response plays a decisive role.
2. Incubation Period
The incubation period is the time between infection and onset of first symptoms.
- Can range from hours to weeks, months, or even years.
- Pathogen reproduces and spreads, often without noticeable signs.
- Many pathogens can already be transmitted during this period, even though the infected person feels healthy.
3. Prodromal Phase
Some diseases show unspecific early symptoms:
- Fatigue
- Mild fever
- Headache, limb pain
- General malaise
These signs often occur before the characteristic symptoms develop and can be confused with harmless ailments.
4. Acute Phase
In the acute phase, symptoms characteristic of the particular disease appear:
- High fever, chills
- Cough, shortness of breath
- Diarrhea, vomiting, abdominal pain
- Skin rashes, spots, blisters
- Pain, swelling, loss of function of affected organs
During this phase, pathogen numbers are often high, and the risk of transmission is usually greatest.
5. Convalescence (Recovery Phase)
If the immune system successfully fights the pathogen, symptoms gradually subside:
- Temperature normalizes
- Appetite and performance return
- Damaged tissues regenerate to varying degrees
The duration of convalescence varies greatly and can be prolonged, especially after severe infections.
6. Possible Outcomes and Special Cases
Infectious diseases do not always “end” in the same way:
- Complete recovery: Symptoms disappear, and the organism returns to its previous state, possibly with acquired immunity.
- Chronic infection: Pathogen remains in the body over a long period; symptoms may be permanent, episodic, or mild.
- Latent infection: Pathogen persists in the body but is inactive for long periods and can reactivate later.
- Carrier state: Infected person has few or no symptoms but continues to shed pathogen and can infect others.
- Complications: Damage to organs, secondary infections, or excessive immune reactions can occur.
- Death: In severe cases, especially without treatment or in vulnerable individuals.
Factors Influencing Susceptibility and Severity
Whether and how severely a person becomes ill depends not only on the pathogen, but also on host and environment factors.
Host Factors
- Age: Infants, young children, and older people are often more susceptible.
- Immune status:
- Previous infections or vaccinations can provide protection.
- Immunodeficiencies (congenital or acquired) increase risk.
- Nutrition: Malnutrition and certain deficiencies weaken defenses.
- Chronic diseases: e.g., diabetes, heart or lung disease, cancer.
- Genetic factors: Some hereditary traits influence susceptibility or resistance.
- Behavioral factors: Hygiene habits, sexual behavior, drug use, travel behavior.
Pathogen Factors
- Dose (infectious dose): Minimum number of pathogens needed to establish infection.
- Virulence: Degree of pathogenicity, i.e., how effectively a pathogen can cause disease.
- Adaptations: Mechanisms that help the pathogen adhere, invade, evade immune responses, or produce toxins.
Environmental and Social Factors
- Population density and crowding: Facilitate spread (e.g., schools, public transport).
- Hygiene and sanitation: Availability of clean water, sewage treatment, handwashing facilities.
- Healthcare access: Diagnostic possibilities, availability of drugs and vaccines.
- Living and working conditions: Ventilation, occupational exposures.
- Global mobility: Travel and trade can rapidly distribute pathogens worldwide.
Contagiousness: Basic Concepts
Some infectious diseases spread far more easily than others. Several concepts help describe this:
- Contagiousness (infectivity): Probability that an infection occurs when a susceptible person is exposed to the pathogen.
- Basic reproduction number $R_0$: Average number of new infections caused by one infected individual in a fully susceptible population.
- If $R_0 > 1$, the disease can spread in the population.
- If $R_0 < 1$, the outbreak tends to die out.
Contagiousness depends on:
- Route of transmission (airborne diseases often more contagious)
- Pathogen survival in the environment
- Duration and level of pathogen shedding
- Human behavior (contact patterns, hygiene)
Prevention and Control: General Principles
Specific preventive measures (e.g., particular vaccines, special medications) belong in the chapters on individual pathogens and on immunobiology. Here, only the general strategies that apply to many infectious diseases are outlined.
1. Breaking Transmission Chains
Measures target the route by which a pathogen spreads:
- Hand hygiene: Washing with soap; use of disinfectants where indicated.
- Respiratory etiquette: Coughing and sneezing into tissues or elbow; wearing masks when appropriate.
- Food safety: Proper cooking, cooling, and storage of food; avoidance of cross‑contamination in the kitchen.
- Safe water and sanitation: Clean drinking water, sewage treatment, toilets.
- Safe sex: Condoms, reduction of high‑risk contacts.
- Vector control: Protection from insects/ticks (repellents, nets, clothing), reducing breeding sites.
2. Protecting Vulnerable Individuals
- Special caution around infants, pregnant individuals, older adults, and people with weakened immune systems.
- Vaccination recommendations often pay particular attention to these groups.
- Infection control in healthcare institutions (e.g., isolation, protective clothing).
3. Vaccination and Immune Protection (Overview)
Immunization is a central tool against many infectious diseases:
- Active immunization stimulates the body to build its own immune protection through exposure to harmless forms of the pathogen or its components.
- Passive immunization involves administration of ready‑made antibodies for rapid, short‑term protection.
Details, mechanisms, and examples of vaccinations are the subject of the chapter on immunobiology and immunization.
4. Diagnosis, Treatment, and Surveillance
- Early diagnosis enables timely treatment and prevents further spread.
- Specific therapies (e.g., antibiotics, antivirals, antiparasitic drugs) can shorten illness duration, reduce complications, and lower infectiousness.
- Public health surveillance records occurrence of infectious diseases and, where necessary, initiates control measures (e.g., contact tracing, vaccinations, temporary isolation).
Infectious Diseases in a Population: Epidemics and Pandemics
When infectious diseases spread beyond individual cases, different terms are used to describe their scale:
- Sporadic: Single, scattered cases.
- Endemic: Constantly present in a region at a relatively stable level.
- Epidemic: A sudden increase in cases above the normal level in a region or population.
- Pandemic: An epidemic that spreads across countries and continents, affecting a large proportion of the global population.
These phenomena reflect interactions between pathogen evolution, human behavior, immunity levels, and public health measures.
This chapter has outlined what defines infectious diseases in humans, how they are transmitted, and which general patterns they follow. The following subchapters will examine specific types of pathogens and representative diseases in more detail.