Keeping your productivity up means staying healthy and energetic. And that can become more of a challenge as the weather becomes chillier.

Reducing your susceptibility to a cold or flu involves maintaining the health of your immune system so it can fight infectious organisms, says Bronwyn Hill, a naturopathic doctor in Toronto.

So, how do you maintain a strong immune system? Hill recommends focusing on the following four components:

1. Vitamin C
Finding ways to incorporate more Vitamin C into your diet is important, whether you are looking to prevent illness or are already sick. It is commonly known that vitamin C is in citrus fruits. But it also can be found in various types of berries, as well as dark, leafy greens and orange and red peppers.

If you enjoy seasonal vegetables, squash is a good way to get some Vitamin C as well, Hill says. Squash can be easily made into soups or added to roasted vegetables.

Hill encourages her patients to keep fresh ginger around the house and add it to foods such as stir-fries, soups and smoothies. “You can do something as simple as making ginger tea,” she says. “Take a couple of slices of fresh ginger root, let it simmer in some water and then you drink it like that.”

Hill suggests keeping ginger in the fridge so it lasts longer.

2. Probiotics
Not all bacteria are bad. The “good” kind play an important part in maintaining your immune system’s balance.

“Unfortunately, through our exposure to foods or toxins, or even due to stress and antibiotic use,” Hill says, “the good bacteria in our gut will get killed off for a variety of reasons.”

Probiotics act as a supplement to help bring those good bacteria into your body. They can be found in dairy products such as cheese and yogurt.

3. Water
Hydration is an important component of immunity. The immune system can be found in your mucous membranes, which includes your nose, mouth, lungs and gut. If these parts of your body are going to defend you from illness, Hill says, they have to be well hydrated. And that means drinking plenty of water.

Water provides another benefit to the immune system by flushing toxins and waste products from our bodies.

4. Sleep
If you boast about getting through the day on just a few hours of sleep, you may want to consider what that lack of rest is doing to your body.

Cutting down on the number of hours you sleep reduces the amount of time your immune system and other bodily systems have to recover.

“If we’re not giving ourselves that time when we’re resting,” Hill says, “the system is going to get behind. It’s like running a car on ‘empty’.”

Further, your body reacts to sleep deprivation as a stressor, which may produce elevated levels of cortisol, a hormone that responds to stress. Cortisol can suppress your immune system.