Q.A 30 year old woman has bleedy diarrhoea for the past 14 hours, which one of the following organisms is likely to cause this illness? a) Streptococcus pyogens b) Clostridium difficile c) Shigella dysenteriae d) Salmonella enteritidis

Although bacterial species are extremely numerous, only a small number are pathogenic to humans; these pathogenic bacteria commonly act by releasing toxins that damage host tissue. Bacterial diseases spread through air, contaminated water, inhaled droplets/aerosols, or shared utensils/clothing with an infected person.

Eight common bacterial diseases are catalogued with their causative agent, site of infection, transmission route and symptoms: shigellosis/bacillary dysentery (Shigella sp., intestine, faecal-oral, bloody mucus-laden stools), bubonic plague (Yersinia pestis, lymph nodes, rat-flea vector), diphtheria (Corynebacterium diphtheriae, larynx/skin/nasal passage, droplet), cholera (Vibrio cholerae, intestine, contaminated food/water), tetanus (Clostridium tetani, muscle spasm, wound infection), typhoid (Salmonella typhi, intestine, contaminated food/water — confirmed via the Widal test), pneumonia (Streptococcus pneumoniae, lungs, droplet), and tuberculosis (Mycobacterium tuberculosis, lungs, droplet).

A key applied concept is antibiotic (bacterial) resistance: overusing an antibiotic, using it for infections the immune system could clear on its own, using it against viral infections, or not completing a prescribed course, all allow some bacteria to evolve resistance — after which that antibiotic can no longer treat that infection effectively. This is why prescriptions should always be followed exactly. Also worth remembering: freezing only arrests bacterial growth, it does not kill bacteria.