Julie Sentman | October 29, 2025

It starts with a sniffle. Then a cough. By mid-October, everyone around you seems to be sneezing! Why does autumn seem to flip a switch on our immune systems? As the leaves turn and temperatures drop, something shifts–not just in the weather, but in our bodies, habits, and in the microscopic world around us. Let’s unpack the science behind the seasonal surge, discover what fall does to our health, and then look at ways to stay healthy this season!

Fig. 1 – Immune cells protect us and prevent the virus from entering our body! (Source: Julie Sentman, with BioRender)

1. Cooler, dryer air

As we transition into autumn, temperatures begin to cool and humidity decreases. This can lead to an increase in virus transmission. Viruses are transferred in microscopic respiratory droplets, which shrink in low humidity and remain airborne longer. In higher humidity, these aerosols may absorb the water from the air, causing them to fall faster [1]. Additionally, lower humidity decreases the effect of disinfectants, thus increasing the prevalence of airborne viruses [2].

Cold air also affects our immune systems, particularly through our nose. When a virus enters the nasal cavity, through breathing or physical touch, our nasal cells release fluid to surround and attack the virus before they can infect our cells. Exposing nasal tissues to cold air decreases nasal temperature and reduces the  immune response in the nose by over 40% [3]. This makes it harder to fight off viruses that enter the nasal passages, increasing our chance of getting sick.

2. Crowding & Increased Exposure

With colder temperatures and the start of the school year, we spend more time indoors. This leads to more close contact with potentially ill individuals, facilitating the spread of germs. Additionally, indoor spaces contain high-touch surfaces, such as doorknobs and countertops, which may harbor germs that survive on these surfaces for varying lengths of time [4]. Along with that, inadequate air ventilation can lead to stagnant air, allowing respiratory droplets to linger in the air which increases our exposure to them [5]. Overall, when indoors, viruses transmit readily due to a higher concentration of airborne viruses. 

3. Reduced sunlight, increased stress, less sleep

In the Northern Hemisphere, days begin to shorten after June 21st, leading to earlier sunsets as fall progresses. These shortened days, combined with spending more time indoors, lead to reduced exposure to the sun and, in turn, lower vitamin D levels. Vitamin D plays a critical role in regulating immune responses and is crucial to almost all immune cells [6]. Low levels of vitamin D are linked to weaker immune responses, which increases susceptibility to infections [7].

This decrease in sunlight also impacts people’s stress levels by impacting serotonin levels and sleep cycles. Inadequate sleep can weaken the immune system, with studies showing that those who routinely get less than seven hours of sleep a night are three times as likely to develop the common cold compared to those who get eight hours of sleep or more [8]. Vaccine studies suggest that those with insufficient sleep prior to and following vaccination develop a significantly lower level of antibodies compared to those who were not sleep deprived. Furthermore, serotonin, a neurotransmitter and hormone involved in many bodily functions, decreases with less sleep, and is crucial for immune responses [9]. Decreased levels of serotonin can impair immune function and increase the risk of infection. 

Fig. 2 –  Your best defenses to autumn ailments! (Source: created by Julie Sentman using Gemini via Google)

Prevention Tips:

In short, autumn is a prime time for getting sick. However, there are ways you can prevent illness from infecting you and spreading to others (see Figure 2)!

1. Humidifiers! Running a clean humidifier can help dampen air and restrict the transmission of the flu.
2. Bundle up! By staying warm through wearing coats, hats, and masks, you can protect your skin and body (and nose!) from the cold temperatures.
3. Vitamin D! By increasing your vitamin D intake you can improve your immune responses! This can be done by taking a vitamin D supplement, adding vitamin D-rich foods to your diet (such as egg yolks, fatty fish, and mushrooms), and spending time outside in the sun when able to.
4. Vitamin C! This antioxidant enhances resistance to infections, and can be ingested through supplements, fruits (especially citrus fruits), and vegetables [10]. However, vitamin C is sensitive to heat, so raw or minimally cooked foods will preserve more vitamin C content.
5. Masks! Not only can masks keep your face warm outdoors, they can also protect yourself and others from airborne viruses indoors!
6. Hydration! Staying hydrated supports the lymphatic system, which transports immune cells to fight infection [11]. 
7. SLEEP! While sleeping, the body produces and releases essential immune cells and proteins. 
8. Take some shots! Of immunity! Your local hospital, university, and drug store offer flu and COVID shots, which are your best way to remain healthy this season!

References:

[1] Wang C.C., Prather K.A., Sznitman J., Jimenez J.L., Lakdawala S.S., Tuffekci Z., Marr L.C. 2021. Airborne transmission of respiratory viruses. Science, 373, 6558. https://doi.org/10.1126/science.abd9149

[2] Mofidfar M., Mehrgardi M.A., Xia Y., Zare R.N. 2024. Depends on relative humidity in the formation of reactive oxygen species in water droplets. PNAS, 121, 12. https://doi.org/10.1073/pnas.2315940121

[3] Huang D., Taha M.S., Nocera A.L., Workman A.D., Amiji M.M., Bleier B.S. 2023. Cold exposure impairs extracellular vesicle swarm-mediated nasal antiviral immunity. Mechanisms of Allergy/Immunology, 151, 2, 509-525. https://doi.org/10.1016/j.jaci.2022.09.037

[4] Cleveland Clinic medical. “Germs: How to Prevent Their Spread.” Cleveland Clinic, March 19, 2025. https://my.clevelandclinic.org/health/articles/24495-germs.

[5] Barron, Madeline. “How Viruses Spread Indoors and What to Do about It.” ASM.org, November 29, 2023. https://asm.org/articles/2023/november/how-viruses-spread-indoors-what-to-do-about-it#:~:text=How%20Do%20Viruses%20Spread%20Indoors,Water%20&%20Energy%20Sustainable%20Technology%20Center.

[6] Aranow C. 2011. Vitamin D and the immune system. Journal of Investigative Medicine, Aug;59(6):881-6. doi: 10.2310/JIM.0b013e31821b8755. PMID: 21527855; PMCID: PMC3166406.

[7] Martens P.J., Gysemans C., Verstuyf A., Mathieu C. 2020. Vitamin D’s Effect on Immune Function. Nutrients, 12, 5, 1248. https://doi.org/10.3390/nu12051248

[8] Chu, Yvonne. “How Sleep Affects Your Immune System > News > Yale Medicine.” How Sleep Affects Your Immune System. Accessed September 26, 2025. https://www.yalemedicine.org/news/how-sleep-affects-immunity.

[9] Wu H., Denna T.H., Storkersen J.N., Gerriets V.A. 2019. Beyond a neurotransmitter: The role of serotonin in inflammation and immunity. Pharmacological Research, 140, 100-114. https://doi.org/10.1016/j.phrs.2018.06.015

[10] Carr A.C., Maggini S. 2017. Vitamin C and Immune Function. Nutrients, 9, 11, 1211. https://doi.org/10.3390/nu9111211

[11] Nagashio S., Ajima K., Maejima D., Sanjo H., Kajihara R., Hayashi M., Watanabe-Asaka T., Kaidoh M., Yokoyama Y., Taki S., Kawai Y., Ohhashi T. 2019. Water intake increases mesenteric lymph flow and the total flux of albumin, long-chain fatty acids, and IL-22 in rats: new concept of adsorption in jejunum. American Journal of Physiology-Gastrointestinal and Liver Physiology, 316, 1, 155-165. https://doi.org/10.1152/ajpgi.00325.2018