Microbiological Food Safety and Foodborne Illness
Bacteria need food, moisture, warmth, time and often oxygen to multiply. Most foodborne bacteria multiply fastest between 5°C and 63°C - the danger zone - and can double roughly every 20 minutes in ideal conditions. Some bacteria form spores (e.g. Clostridium perfringens, Bacillus cereus) which survive cooking; others produce toxins (e.g. Staphylococcus aureus) that are not destroyed by reheating.
Know the main organisms and their typical sources: Salmonella (raw poultry, eggs), Campylobacter (raw poultry, cross-contamination), E. coli O157 (raw meat, unpasteurised products), Listeria (chilled ready-to-eat foods, soft cheese, pâté - dangerous in pregnancy), Staphylococcus aureus (hands, skin, nose), Clostridium perfringens (bulk-cooked meat, stews, gravies cooled slowly), and Bacillus cereus (rice and pasta). Norovirus is a virus spread by infected handlers and contaminated surfaces.
Traps: candidates confuse intoxication (toxin already in food, rapid onset) with infection (bacteria multiply in the gut, longer onset). Listeria is often the answer for chilled RTE food and vulnerable groups. Reheating does not make toxin-containing food safe.
Rule of thumb: for high-risk foods (meat, dairy, eggs, cooked rice, shellfish), keep them out of the danger zone as much as possible - cold below 5°C or hot above 63°C.