Specifically how did Mary Mallon spread typhoid bacteria (Salmonella typhi) to the families she cooked for, as cooking food kills bacteria

Mary Mallon was able to spread the typhoid bacteria to the families she cooked for even as cooking kills bacteria because she was a healthy carrier of salmonella typhi who prepared certain foods for these families, that doesn't require cooking, without vigorously scrubbing her hands.

Explanation:

The genus salmonella consists of a group of bacilli which can affect man leading to the following conditions:

--> Enteric Fever

--> Gastroenteritis

--> Septicaemia and

--> Carrier state or condition.

Salmonella typi causes the enteric fever called TYPHOID FEVER. A healthy individual can be infected with the bacteria through injection of faecal contaminated food ( faeco- oral route). On reaching the gut,the organism attach themselves to the intestinal villi penetrating the lamina propria and submucosa. Their major factor of virulence is the inability to be killed after being phagocytosed by polymorphs and macrophages. They enter the blood stream through the thoracic duct causing transient bacteriaemia. During this period, the bacilli are then seeded into the liver, kidney and gall bladder where further multiplication occurs. The massive discharge of these bacilli from gall bladder leads to onset of the clinical disease.

These bacilli tends to persist in the kidney and gall bladder( as it's a good culture media for the organism). These are seen in individuals who become carriers following inapparent infection and they are called HEALTHY CARRIERS. They shed the bacilli intermittently from their faeces and urine

Mary Mallon, an Irish cook, was a healthy carrier of salmonella typhi who constantly shed the bacteria in her faeces and urine. Although the bacilli is destroyed when subjected to high temperature ( when cooking), the families she cooked for still came down with the illness. This was so because, since the bacteria can be transmitted through faeco- oral route, she wasn't educated enough to vigorously scrub her hands before handling foods that doesn't require cooking,for example vegetable salads.