Why we get sick more in the fall and winter
In September the R comes in the month and the days get shorter. In this month, autumn starts, it gets colder and it rains and storms more often. Suddenly you see everyone sniveling, coughing and sneezing. The autumn and winter months are characterized by colds and flu. But how come we get sick so quickly in these seasons?
When it is cold outside we are often indoors
Some studies suggest that both the cold air from outside and the dry air from inside can play a role in protecting the aerosol droplets that we sneeze and cough in the air, making it easier for them to spread from one sick person to another.
Plus, clogged, unventilated indoor air can make it easier for a cold to spread; a 2011 study of busy university halls in China showed that in rooms with poorer ventilation, a cold would sooner thrive.
Bacteria thrive better in cold weather
Some studies by the National Institutes of Health suggest that the outer shell of influenza virus particles becomes thicker and hardy at cold temperatures, so that it survives longer and is easier to spread.
And being outside when it’s cold can make it harder for the hair and mucus in our nose to protect us from germs. A study of mice published last year showed that rhinovirus, which causes the common cold, replicates more easily at lower temperatures than at warmer ones.
Our genes change with the seasons, just like the weather
A recent study found that no less than a quarter of our DNA actually changes with the seasons: during the winter months, the study found, our bodies pump up the levels of many of the genes related to inflammation, causing the signs of swelling and discomfort that our body uses to protect us from colds and flu are triggered.
In the summer, on the other hand, an entirely different set of genes is more appealed to, including some that help regulate our blood sugar levels, potentially reduce cravings, and help us burn excess fat.
Many parts of our immune system, which comes into effect to avert an infection or cold, are also shifting. The researchers searched data from previous human DNA studies until they had information about about 1,000 people living in six different countries: Australia, Germany, the US, the UK, Iceland and The Gambia.
In this way, they could take a look at people’s genes and how they changed (if they did) about their location and exposure to sunlight.
They discovered that in Europe the expression of inflammatory genes increased during the winter months. But in Gambia, where there is virtually no winter, these inflammatory genes were amplified in the rainy months, when mosquito populations are at their peak and the risk of malaria is greatest.
Previous research has found similar seasonal changes in various components of the immune system. A study last year, for example, found that gene expression in red blood cells shifted with the seasons.
- “Widespread seasonal gene expression reveals annual differences in human immunity and physiology” Xaquin Castro Dopico, Marina Evangelou, Ricardo C. Ferreira, Hui Guo, Marcin L. Pekalski, Deborah J. Smyth, Nicholas Cooper, Oliver S. Burren, Anthony J. Fulford, Branwen J. Hennig, Andrew M. Prentice, Anette-G. Ziegler, Ezio Bonifacio, Chris Wallace & John A. Todd Nature Communications volume6, Article number: 7000 (2015)
- Simone De Jong, Marjolein Neeleman, Jurjen J. Luykx, Maarten J. Ten Berg, Eric Strengman, Hanneke H. Den Breeijen, Leon C. Stijvers, Jacobine E. Buizer-Voskamp, Steven C. Bakker, René S. Kahn, Steve Horvath, Wouter W. Van Solinge, Roel A. Ophoff, Seasonal changes in gene expression represent cell-type composition in whole blood, Human Molecular Genetics, Volume 23, Issue 10, 15 May 2014, Pages 2721–2728, https://doi.org/10.1093/hmg/ddt665
- Sun Y, Wang Z, Zhang Y, Sundell J. In China, students in crowded dormitories with a low ventilation rate have more common colds: evidence for airborne transmission. PLoS One. 2011;6(11):e27140. doi:10.1371/journal.pone.0027140
- Temperature-dependent innate control of rhinovirus, Ellen F. Foxman, James A. Storer, Megan E. Fitzgerald, Bethany R. Wasik, Lin Hou, Hongyu Zhao, Paul E. Turner, Anna Marie Pyle, Akiko Iwasaki, Proceedings of the National Academy of Sciences Jan 2015, 112 (3) 827-832; DOI: 10.1073/pnas.1411030112