Infection Types: Active vs. Latent Primary Infection

• Asymptomatic infection: An infection showing no symptoms.
• Primary infection: Initial infection of a host by a pathogen.
• Secondary infection: Activation of a latent infection or a second stage of an infection.
• Mixed infection: Infection of a tissue by two or more bacteria.

Active Primary Infection

• An infection currently causing symptoms or disease, with the pathogen actively multiplying and causing damage.
• Example: Chickenpox, caused by the Varicella zoster virus, manifests with fever and rash.

Latent Primary Infection

• After the initial infection, the pathogen enters a dormant state, no longer causing symptoms but remains in the body.

• The pathogen may stay hidden for months, years, or even a lifetime.

• Example: After recovering from chickenpox, the Varicella zoster virus becomes latent, hiding in nerve cells without causing any symptoms.

• Primary infection is the first encounter with a pathogen, whether active or latent.

• A latent primary infection can reactivate and cause additional disease manifestations, like shingles from the Varicella zoster virus.

Bacterial Pathogenesis

• Bacterial pathogenesis: The process by which bacteria infect and cause disease in a host.
• The infectious process starts with microorganisms entering the host through routes like inhalation, oral, urogenital, and rectal.
• Microbes adhere to host cells using organelles like pili and adhesions to glycolipids on host membranes.
• After adhering, the next step is invasion through host barriers or cells to spread the infection from the initial site.
• Once inside, bacteria propagate and produce toxins and virulence factors, damaging host cells.
• Host cells are damaged due to toxins, virulence factors, or the host's immune response.
• Bacteria continue to infect host tissue or are eliminated by the host's immune response or antimicrobial treatment.

Virulence Factors

• Virulence factors are bacteria-associated molecules required for bacteria to cause disease by infecting eukaryotic hosts.

• Examples: Capsule salval proteins, Edison invasins, degraded enzymes, and bacterial toxins.

• Streptococcus pneumoniae: Polysaccharide capsule

• Bacillus anthracis: Edema toxin

• Streptococcus pyogenes: M protein

• Staphylococcus aureus: Protein A on the cell wall

Protein A (Staphylococcus aureus)

• Evades immune detection by binding to the Fc region of immunoglobulins (antibodies), preventing immune cells from recognizing the bacteria.
• Prevents phagocytosis, stopping immune cells from engulfing and destroying the bacteria.
• Makes S. aureus resistant to immune attacks, allowing the bacteria to grow and cause infections.

M Proteins (Streptococcus pyogenes)

• Inhibits phagocytosis, interfering with the ability of immune cells to engulf the bacteria.
• Prevents complement activation, blocking the immune system from attacking bacteria.
• Enhances adhesion, helping the bacteria stick to host cells, making it easier to establish infection.
• Helps S. pyogenes avoid immune defenses and spread through tissues, contributing to diseases like strep throat, scarlet fever, and rheumatic fever.

Viral Pathogenesis

• Viral pathogenesis: The process and mechanisms by which viruses cause diseases in their target hosts.
• Because viruses can replicate only inside living cells, pathogenic manifestations are seen at the cellular level.
• Adenovirus and Coronavirus infections cause cell death.
• HPV infection results in transformation of the whole cell into a cancer cell.
• Measles virus and paroxyvirus infections cause cell fusion.

Cytopathic Effect

• Cytopathic effect: Structural changes in a host cell resulting from viral infection.
• Cytopathic effects are useful for diagnosing viral infections.
• Coronaviruses: Nuclear shrinking
• Polyomaviruses, Papillomavirus: Vacuoles in cytoplasm
• Paramyxoviruses, Coronaviruses, Respiratory syncytial virus: Syncytia formation
• Various viruses: Virions in cytoplasm, like medical bodies