1) Why do some patients still develop gastroenteritis after eating days-old food

1) Why do some patients still develop gastroenteritis after eating days-old food despite reheating?
2) What are some parasites associated with eating raw meats (pork and beef), and describe briefly their clinical presentations
3) What is the difference between community-acquired pneumonia and hospital-acquired pneumonia? (include the most common pathogens, and complications)