Infectious Diseases and Pathogens Overview

Infectious and Parasitic Diseases

• Microorganisms (bacteria, viruses, fungi, and parasites) are ubiquitous in soil, water, air, and on surfaces and cause many infectious diseases in humans.

Chain in Transmission of Infectious Diseases

• Reservoir of pathogen
• Route of infection
• Mode of transmission
• Susceptible host

Mode of Entry

• Microorganisms enter the body through ingestion, inoculation (parenteral), inhalation, perinatally, direct contact, or from contaminated sources (water, food, soil, or animals).

Factors Relating to Host

• Physical barriers: intact skin and mucosa prevent entry.
• Chemical barriers: mucus and stomach acid.
• Effective drainage: respiratory and gastrointestinal tracts.
• Immune defenses: phagocytic leukocytes and the immune system.

Diseases Caused by Bacteria, Spirochaetes, and Mycobacteria

• Plague is caused by Yersinia (Pasteurella) pestis.
  • It's a zoonotic disease, primarily spread by rats via rat fleas or inhalation.
  • Characterized by rapid progression, chills, fever, myalgia, nausea, vomiting, marked prostration, disseminated intravascular coagulation (DIC), widespread petechiae and ecchymoses, and potential gangrene (black death).
• Anthrax is caused by Bacillus anthracis.
  • It's a zoonotic disease that is common in animals (cattle and sheep) but rare in humans.
  • Usually an exogenous infection, contracted through contact with soil or animal products contaminated with spores.
  • Three forms: cutaneous, pulmonary, and gastrointestinal. Pulmonary anthrax is most dangerous.
• Whooping cough (Pertussis) is caused by Bordetella pertussis.
  • Highly communicable disease.
  • Causes low-grade fever, rhinorrhoea, conjunctivitis, excess tear production.
  • Characterized by paroxysms of coughing with a whooping sound; may be fatal in infants due to asphyxia.

Staphylococcal Infections

• Staphylococci are gram-positive cocci found in skin, umbilicus, nasal passages, and stool.
• Staph. aureus is the most common pathogen in staphylococcal infections.
• Staphylococcal infections include skin infections, burn infections, respiratory tract infections, bone infections (osteomyelitis), bacterial endocarditis, bacterial meningitis, sepsis, and toxic shock syndrome.

Streptococcal Infections

• Streptococci are gram-positive cocci that are more frequently associated with non-suppurative complications and autoimmune reactions..
• Group A streptococci (Streptococcus pyogenes), responsible for upper respiratory tract infections and skin infections.
• Group B streptococci (Streptococcus agalactiae), a major cause of neonatal infections.
• Group C and G, responsible for respiratory infections.
• Group D streptococci (Streptococcus faecalis, also called enterococci), can cause urinary tract infections, bacterial endocarditis, and sepsis.
• Less common streptococci, such as Streptococcus viridans and Streptococcus pneumoniae, can lead to bacterial endocarditis and pneumonia respectively.

Clostridial Diseases

• Clostridia are gram-positive, spore-forming, anaerobic microorganisms.
• Common diseases: gas gangrene (caused by C. perfringens), tetanus (caused by C. tetani), botulism (caused by C. botulinum), clostridial food poisoning (caused by C. perfringens), and necrotizing enterocolitis (caused by C. perfringens).

Diseases Caused by Fungi

• Mycetoma is a chronic, suppurative infection characterized by draining sinuses with grains (fungal or bacterial colonies).
  • Two types: actinomycotic (higher bacteria) and eumycetomas (true fungi).
• Candidiasis is an opportunistic fungal infection caused most commonly by Candida albicans.
  • Present as normal flora in the skin, mucous membranes, intestines, and vagina.
  • Pathogenic when the host-organism balance is disturbed.

Diseases Caused by Viruses

• Viral diseases are a common cause of human illness.
• Viruses are intracellular parasites with single- or double-stranded RNA or DNA genomes.

Yellow Fever

• The oldest known viral hemorrhagic fever, restricted to Africa and South America.
• Characterized by: high fever, chills, myalgia, headache, jaundice, hepatic failure, renal failure, bleeding disorders, and hypotension.

Dengue Hemorrhagic Fever (DHF)

• Caused by virus transmitted by the mosquito Aedes aegypti.
• Two forms:
  • Dengue fever ("break-bone fever")—a self-limited febrile illness affecting muscles/joints with severe back pain.
  • Dengue hemorrhagic fever (DHF)—more severe and life-threatening. Characterized by vascular permeability, thrombocytopenia, plasma leakage, and potentially fatal shock.

Varicella-Zoster Virus

• Causes chickenpox (varicella) and shingles (herpes zoster).
• Chickenpox is an acute vesicular exanthem (skin rash).
• Shingles is a reactivation of dormant varicella zoster virus.

Herpes Simplex Virus

• Two types: HSV-1 (oral herpes) and HSV-2 (genital herpes).
• Characterized by vesicular lesions on skin, mouth, and genitals.

Rabies

• Fatal encephalitis caused by rabies virus.
• Transmitted through the bite of infected mammals.
• Characterized by a long incubation period, followed by progressive neurological symptoms (difficulty swallowing, painful spasms (hydrophobia), and delirium) that lead to death within weeks of infection.

Amoebiasis

• Caused by Entamoeba histolytica.
• Exists in two forms: trophozoite (active form in tissues and stool) and cyst (resistant form that survives outside the body)

Malaria

• Caused by Plasmodium species (e.g., P. falciparum, P. vivax).
• Transmitted through the bite of infected female Anopheles mosquitoes.
• Can cause severe complications including cerebral malaria, a major cause of death.