I read in a PubMed study that a relatively high percentage of people who get the flu don’t actually have any symptoms, but they still shed virions. (I don’t remember exactly which study or the actual figures.) They are what’s called asymptomatic carriers. Getting a flu shot can increase your chances of becoming an asymptomatic carrier, which means that you could get others sick, because by not knowing you have the virus in your system, you won’t be taking the necessary precautions to prevent contaminating others. That is a big issue especially if you will be around immunocompromised people. Washing your hands and staying away from others when sick is the single best way to prevent the spread of communicable diseases.