The specifics of infection and infectious diseases. The course of infectious diseases in the host organism and in the population. Koch’s postulates. The food chain and the One Health concept.

Harmless or Beneficial Microbes

Apathogenic microbes

Microbes Dangerous to Us

• Pathogenic microbes
• Obligate pathogens
• Facultative pathogens
• Opportunistic pathogens
• Toxin-producing microorganisms– the toxic factors of virulence

Pathogenic microorganism

A living organism or entity capable of causing disease in an individual of another species.

Pathogen

infectious agent or disease-causing organism

Pathogenicity

the ability to cause disease, referring to the potential of a specific species to cause illness in a host population

Infectivity

the ability to infect a host

Virulence

the degree or severity of the disease caused

Interactions of the microbe and the host

• Mutualism
• Commensalism
• Parasitism

The interaction influenced by

• The host species
• The current immune status of the host
• The localization of the microbe within the host (danger: inappropriate location in the host)

Principles of pathogenicity

• Pathogenicity indicates whether the microbe may cause disease in that particular host species
• Virulence: the measurable disease-causing ability of a microbe; LD₅₀ (Lethal Dose 50%); ID₅₀ (Infectious Dose 50%); TCID₅₀ (Tissue Culture Infectious Dose 50%)
• Virulence factor: a microbial molecule, structure or function determining virulence
• Attenuation: producing strains with decreased virulence (vaccines)

Koch- postulates (germ to disease)

1. The organism must be present in every case of the disease

2. The organism must be isolated from the host, continuing the disease in vitro

3. Samples of the organism must cause the same disease when inoculated into a healthy animal

4. The organism must be isolated from the inoculated animal and must be identified as the same OG organism that was originally separated from the host

The specifics of infectious diseases

1. Infection

2. Important characteristics of pathogenic microbes - infectious dose, virulence

3. Exogenous sources
   - Carrier individual
   - Diseased individual with symptoms
   - Environmental factors (e.g., food, water, soil, air)
   - Reservoirs: biotic and abiotic factors (e.g., human and animal hosts, water systems) where the pathogen can persistently exist, multiply, and survive in infectious form

4. Endogenous sources – member of the normal flora, activation of persistent microbes

5. Transmission mode:
   - Horizontal transmission – occurs between individuals at the same time and place, within the same generation (e.g., contact, environment– foodborne infections, contaminated water, vectors, non-sterile instruments healthcare-associated infections)
   - Vertical transmission - from one generation to the next

6. Portals of entry (respiratory tract, gastrointestinal tract, urogenital tract, skin and mucous membranes, across the placenta)

7. Modes of spread- shedding

8. Susceptibility of the host

Course of infection in the host

1. Based on clinical symptoms
   • asymptomatic or subclinical → carrier status
   • Manifest → later symptoms

2. Based on temporal course
   • Acute
   • Subacute
   • Chronic
   • Latent
   • Recurrent

3. Stages of acute infection
   • Incubation period – latent stage
   • Prodromal stage – initial phase
   • Acute phase – appearance of characteristic symptoms
   • Convalescence– recovery, healing