Infectious & Environmental Diseases

• Groups vulnerable to infectious diseases

  • Elderly
  • Immunosuppressed
  • Chronic disease states
• 6/10 causes of mortality = by malnutrition & unsanitary living conditions
• Pathologic infection

  Damage of host defense/microbial flora
• Highly infectious microbes

  Cause disease in healthy individuals
• Routes of Entry of Microbes

  • Skin
  • GI tract
  • Respiratory tract
  • Urogenital tract
• Vertical transmission - mom to fetus/newborn

  • Placental-fetal transmission during pregnancy (1st trimester = rubella, 2nd trimester = SARS Covid)
  • Transmission during birth (Chlamydia)
  • Postnatal transmission in maternal milk (HIV, CMV)
• Release From the Body & Transmission of Microbes


  • Person to person
  • Animal to human
  • Insect or arthropod vectors
  • Survival in dust, food, or water
• Host Defences Against Infection

  • Innate and adaptive immune systems fail to protect against infections due to microbial latency, leading to aggressive reactivation/loss of host and disease like EBV and TB
• Immune Evasion by Microbes

  • Microbes can evade immunity through various mechanisms, including resistance to antimicrobial peptides, phagocyte killing, apoptosis, and manipulation of host cell metabolism. They also resist cytokine-/chemokine-/complement-mediated host defense, and are recognized by CD4+ helper T cells and CD8+ cytotoxic T-cells. T-cells respond by utilizing immunoregulatory mechanisms, and they 'ly low' until viral genes are expressed, resulting in latent infection
• Injurious Effects of Host Immunity

  • Granulomatous responses can hide pathogens and cause tissue damage, liver damage, and immune destruction in infected hepatocytes. Cross-reactions with antibodies and chronic inflammation can lead to malignancy, such as Helicobacter pylori and gastric cancer
• Genetic immunodeficiencies

  • Antibody deficiencies
  • Complement protein deficiencies
  • Neutrophil function deficiencies
  • T-cell deficiencies
• Acquired immunodeficiencies

  HIV annihilation of T-helper cells
• Acquired immunodeficiencies

  • AIDS before HIV-AIDS
  • Kaposi sarcoma herpes virus [KSHV], cryptococcus, & Pneumocystis (uncommon)
  • Malnutrition
• Immunosuppression


  • Immunosuppressive drugs
  • Transplantation
  • Bone marrow engraftment
  • Hematopoietic stem cell transplant
• Diseases of organs other than the immune system that render patients susceptible to specific organisms

  • Lack of splenic function in sickle cell disease (encapsulated bacteria)
  • Respiratory infections with Pseudomonas in cystic fibrosis
  • Exposure to microbes in burns (P. aeruginosa)
• Infectious disease


  • Interaction of microbial virulence characteristics & host immune responses
• Mechanisms of Viral Injury

  • Tropisms
  • Direct cytopathic effects
  • Inducing antiviral host immune response
  • Cytotoxic T cells/natural killer (NK) cells
  • Transformation of infected cells
• Mechanisms of Bacterial Injury

  • Bacterial Virulence
  • Bacterial Adherence to Host Cells
  • Virulence of Intracellular Bacteria
  • Bacterial Toxins
• Endotoxins (lipopolysaccharide [LPS])


  Lipid A, a long-chain fatty acid and carbohydrate chain, provides protective inflammatory cell recruitment and cytokine production in low doses, while higher doses can cause septic shock, intravascular coagulation, and adult respiratory distress syndrome
• Exotoxins

  Damage host tissues by digesting structural proteins
• Neurotoxins

  Block neurotransmitter release = paralysis (e.g., in botulism & tetanus)
• Superantigens

  The activation of T cells by linking class II MHC molecules on APC leads to massive T-cell proliferation and cytokine release, such as toxic shock syndrome caused by S. aureus
• Risk groups for Sexually Transmitted Infections

  • Adolescents
  • Homosexuals
  • IV drug users
• STIs in children = sexual abuse (not at birth)
• STI in pregnant spread to the fetus in utero or at delivery = severe damage
• Spectrum of Inflammatory Responses to Infection

  • Suppurative (Purulent) Inflammation
  • Mononuclear & Granulomatous Inflammation
  • Cytopathic-Cytoproliferative Reaction
  • Tissue Necrosis
  • Chronic Inflammation & Scarring
• Special Techniques for Diagnosing Infectious Agents

  • Routine H&E staining
  • Special stains
  • Cultures of fluids and tissues
  • Antibody titers
  • Nucleic acid amplification tests
• Viral Infections - Acute (Transient) Infections

  • Measles
  • Mumps
  • Poliovirus Infection
  • West Nile Virus
  • Viral Hemorrhagic Fever (Ebola, Marburg & Lassa)
• Latency

  Persistence of viral genomes without infecting, causing dissemination and tissue injury
• Human herpes viruses

  • α-Group (HSV-1)
  • β-Group (CMV)
  • γ-Group (EBV)
• Herpes Simplex Viruses

  • Herpes replicates in skin and mucous membranes
  • Causes latent infection
  • Lesions range from cold sores to life-threatening disseminated visceral infections
  • Herpes epithelial keratitis
  • Herpes stromal keratitis
• Varicella-Zoster Virus

  • Transmitted by aerosols
  • Causes chickenpox, shingles, and herpes zoster
  • Infects mucous membranes, skin, and neurons
  • Causes life-threatening diseases in immunocompromised hosts
• Cytomegalovirus infection

  • Transmitted through breast milk, respiratory droplets, blood, and saliva
  • Can cause life-threatening diseases like colitis, hepatitis, chorioretinitis, and meningoencephalitis in AIDS/immunosuppressed patients
  • Causes cytomegalic inclusion disease (CID), intrauterine growth retardation, hemolytic anemia, jaundice, encephalitis, deafness, and mental retardation
• Viral Infections - Chronic Productive Infections

  Immune system struggles to eliminate the virus, leading to persistent viremia. High mutation rates may be a defense mechanism (e.g. HIV, HBV)
• Viral Infections - Transforming Viral Infections

  • Epstein-Barr Virus
• Epstein-Barr Virus

  • Occurs through close contact, such as saliva, blood, or venereal transmission
  • Begins in nasopharyngeal and oropharyngeal epithelial cells
  • Infects B cells in underlying lymphoid tissues
  • Establishes a latent infection via genes that induce B-cell proliferation and production of heterophile antibodies
  • Causes infectious mononucleosis
• Cytomegalic inclusion disease (CID)


  Caused by CMV infection, results in marked cellular enlargement
• CMV infection causes cytomegalic inclusion disease (CID), intrauterine growth retardation, hemolytic anemia, jaundice, encephalitis, deafness, and mental retardation
• Chronic Productive Infections

  In certain infections, the immune system struggles to eliminate the virus, leading to persistent viremia. High mutation rates may be a defense mechanism. e.g. HIV, HBV
• Transforming Viral Infections

  Viral infections that can transform or alter the host cells