Flu season is almost upon us and there’s a flu shot out there with your name on it. But have you ever wondered why, in the US, we get our flu shots annually in the fall and why the flu is more common in the colder months? Fortune recently reported on basic flu facts to help answer your questions:

• When is the flu season? The US flu season usually peaks between December and February, but can start as early as October and run all the way through May.
• When is the best time to get a flu shot? It can take up to two weeks after vaccination for your body to develop antibodies against the flu virus. So, aim to get your shot before the season starts for the most protection.
• Is getting a flu shot every year really necessary? The most prevalent flu virus strains change from year to year, so getting your yearly shot gives you a better chance of protection against the latest mutations.
• Who should get the flu shot? Everyone over the age of six months, even if you are pregnant or have a compromised immune system.
• How effective is the flu shot? Its effectiveness varies widely, but the CDC estimates the flu shot reduces an individual’s risk of catching the flu by 40 to 60%.
• Why does flu season occur during the colder months? While the answer to this is unclear, there are a few theories: one is that lower humidity and colder temperatures help the virus thrive and travel through the air in a transmissible form; another proposes that fewer UV rays, which may kill the virus, and colder temperatures allow it to persist in the air and on surfaces. And, of course, cold weather drives people indoors in closer quarters where germs spread more easily.

Source: Fortune and CDC