Physical contact Trichophyton Bacillus anthracis Clostridium tetani Francisella tularensis Athlete’s foot Mosquito bites (Aedes aegypti) Deer tick bites Mosquito bites (Anopheles) Flavivirus Borrelia burgdorferi Plasmodium spp. Yellow fever Lyme disease Malaria Minor skin abrasions Puncture wounds Handling infected animals Cutaneous anthrax Tetanus Tularemia Route of entry Mode of transmission Pathogen Disease External epithelia Mucosal surfaces Routes of infection for pathogens Mouth and respiratory tract Contaminated water or food Sexual transmission/ infected blood Sexual transmission Neisseria meningitidis Bacillus anthracis Meningococcal meningitis Inhalation anthrax Fungus Virus Bacterium (spirochete) Protozoan Gram-positive bacterium Gram-positive bacterium Gram-negative bacterium Type of pathogen Hepatitis B virus Hepatitis B Hepadnavirus Neisseria gonorrhoeae Gonorrhea Gram-negative bacterium Human immunodefciency virus (HIV) Acquired immunodefciency syndrome (AIDS) Retrovirus Gram-negative bacterium Gram-positive bacterium Wounds and abrasions Insect bites Gastrointestinal tract Reproductive tract and other routes External surface Resident microbiota Candida albicans Candidiasis, thrush Opportunistic infections Fungus Resident lung microbiota Pneumocystis jirovecii Pneumonia Fungus Measles virus Measles Paramyxovirus Infuenza virus Infuenza Orthomyxovirus Varicella-zoster Chickenpox Herpesvirus Epstein–Barr virus Mononucleosis Herpesvirus Streptococcus pyogenes Tonsillitis Gram-positive bacterium Haemophilus infuenzae Pneumonia, meningitis Gram-negative bacterium Rotavirus Diarrhea Rotavirus Hepatitis A Jaundice Picornavirus Salmonella enteritidis, Food poisoning Gram-negative bacteriumS. typhimurium Vibrio cholerae Cholera Gram-negative bacterium Inhalation or ingestion of infective material (e.g., saliva droplets) Salmonella typhi Trichuris trichiura Typhoid fever Trichuriasis Gram-negative bacterium Helminth Treponema pallidum Syphilis Bacterium (spirochete) Spores
Fig. 2.2 A variety of microorganisms can cause disease. Pathogenic organisms are of five main types: viruses, bacteria, fungi, protozoa, and worms. Some well-known pathogens are listed.
Virtually all pathogens have an extracellular phase in which they are vulnerable to the circulating molecules and cells of innate immunity and to the antibodies of the adaptive immune response. All these clear the microorganism mainly by promoting its uptake and destruction by the phagocytes of the immune system. Intracellular phases of pathogens such as viruses are not accessible to these mechanisms; instead, the infected cell is attacked by the NK cells of innate immunity or by the cytotoxic T cells of adaptive immunity. Activation of macrophages as a result of NK-cell or T-cell activity can induce the macrophage to kill pathogens that are living inside macrophage vesicles.