Yes. As with all infectious diseases, some people are more likely to catch the disease, while others are less likely to. A person who has close contact with many other people will be exposed to more sick people, and will be more likely to catch the flu. Someone who has no protection from past flu vaccines or flu illnesses, will more easily get the disease after exposure to the virus.
Yes. The flu is spread from person to person through microscopic droplets expelled during coughing or sneezing, or if you inadvertently touch a surface or an object covered with those droplets. As such, people in close contact with a sick individual are more likely to get the flu. This includes babies and infants in daycare centers and older children in school, and t heir family members.
Yes. In my experience this does not apply to just the flu. Some people are more prone to get sick. If one is overworked, overtired, eats a diet rich in junk food and poor in fruits and veggies, the immune system is affected. People get sick when their immune system is "down". Vitamin d can help improve immunity as can a god night's sleep, a diet full of whole grains, fruits, veggies, and fresh food.