As Robert said and I know full well being a nurse practitioner, there is no way that you'll be able to drag yourself to church if you truly have the flu. It puts you down!! People confuse a "bad cold" or bronchitis with the flu. Flu keeps you out of circulation for days.
As others already said, cover when you sneeze and cough, sit away from others, don't shake hands, wash your hands often!!! and call or visit your primary care provider for guidance on your symptoms and what you have.
Ask your priest for guidance on whether it's a sin when you have a slight illness.