Chapter 31: Person-to-Person Bacterial and Viral Diseases

Airborne bacterial infections include illnesses caused by respiratory pathogens such as Streptococcus pyogenes, which produces streptococcal pharyngitis and complications like scarlet fever and rheumatic fever, and Streptococcus pneumoniae, a major cause of pneumonia. Other important respiratory diseases include diphtheria caused by Corynebacterium diphtheriae, pertussis produced by Bordetella pertussis, and tuberculosis caused by Mycobacterium tuberculosis, which forms lung lesions known as tubercles and is controlled by cell mediated immunity. Viral airborne diseases include measles, mumps, rubella, and varicella infections that spread through respiratory droplets and are largely prevented through vaccination programs such as the MMR vaccine. Influenza represents another highly contagious respiratory infection whose epidemiology is shaped by antigenic drift and antigenic shift, genetic changes that alter viral surface proteins and require frequent vaccine updates. The chapter also discusses infections transmitted through direct contact, including Staphylococcus aureus infections that produce skin lesions, toxic shock syndrome, and antibiotic resistant strains such as MRSA, as well as Helicobacter pylori, a bacterium associated with gastric ulcers and stomach cancer. Viral diseases such as hepatitis, Ebola virus disease, and common cold infections demonstrate additional modes of direct or bodily fluid transmission. Sexually transmitted infections represent another major category of person to person disease, including bacterial infections such as gonorrhea, syphilis, and chlamydia, along with viral infections such as herpes simplex virus, human papillomavirus, and human immunodeficiency virus, the causative agent of AIDS. Together these diseases illustrate how microbial pathogens exploit different transmission pathways, host tissues, and virulence mechanisms to spread within human populations and produce a wide range of clinical outcomes.