Updated November 14, 2014.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
To everything there is a season
Infectious Diseases are no different. Certain illnesses spread more during different seasons.
Fall and Autumn
Kids can spread diseases. They don't wash their hands. They touch things. They put their hands in their mouths. They touch each other. They sneeze and don't think about tissues. They may get sicker because their immune systems haven't seen as many bugs as the rest of us.
They may have high rates of "disease shedding"; they have higher viral levels making them more infectious, because their immune systems don't yet know how to hold back these infections. They may also carry more bugs even if they aren't sick, like a bacteria that causes pneumonia.
Back-to-school can mean some infections spread more. Human Rhinovirus Virus which causes colds spreads among kids in the fall. This virus can make it a lot harder for asthmatic kids to breathe. There is a peak in hospitalizations of children with asthma in September in the US. This is likely the case with Enterovirus-D68, a virus much like a Rhinovirus, spreading in September 2014.
The back-to-school effect likely also affected H1N1spread during the SwineFluepidemic.
Cold and flu season runs throughout the winter. We are indoors breathing in the same small spaces - our classrooms, our work, our homes. We have more contact; each cough and sneeze can spread more.
It might be more than just us being inside which spreads these viruses.
Influenza also appears to spread best in dry air. Cold air doesn't hold as much humidity as warm air. Indoor heating may just warm the air, but without a humidifier, still leave air dry.
Some viruses last longer when the temperature is cooler, evenfreezing. Norovirus - the winter vomiting bug - spreads in colder winter weather. It can persist in cold food and on objects, called fomites, that we touch, like tabletops and doorknobs, and that can spread the infection to others.
Some also worry that lower levels of vitamin D in the winter due to less sunshine may increase our risk for infections. It may make pneumonia more severe. It may affect Tuberculosistransmission.
Allergies are often mistaken for infections in the spring. "Pink Eye" or a "cold" may not be an infection at all, but an allergy to pollen in the spring.
There are also some infections that spread in the spring once the flu and cold season ends, such as parainfluenza which causes respiratory illness.
Salmonella can even spread if chickens and raw eggs are handled at Easter time.
In the summer, the bugs come out - the actual bugs, mosquitoes and ticks. There are mosquitoes that spread infections, causing West Nile Virus to spread in the US - and in Florida now, Dengue and Chikungunya. As the mosquitoes die out as it gets colder, the infections die out too. Avoiding mosquitoes, wearing DEET, covering up, and getting rid of open water for mosquito breeding is important.
We also pick up ticks in the summer. These can lead to Lyme disease, Babesia, Ehrlichiosis, as well as the very dangerous Rocky Mountain Spotted Fever. To avoid these: wear clothes to avoid exposed skin (especially ankles and legs) as well as using DEET and removing ticks from pets and clothing.
We also spend a lot more time in the water. Some bugs are spread by swimming or from fountains in the summer time: cryptosporidiosis, giardiasis, or shigellosis, to name a few.
We also eat outdoors. Summer picnics and barbeques, especially in warm weather when bacteria can grow quickly, can result in food poisoning. Likewise, although norovirus is common in the winter, some person-to-person gastrointestinal bugs caused by bacteria, like Shigella, can spread in the summer.
Where the seasons are different.
Where seasons are different, diseases spread at different times. The influenza season in Australia is almost half a year different from the US (May through October in Australia, while the season starts in October in the US). Areas around the equator may not much seasonal variation - and see influenza throughout the year.
Other diseases in more Tropical Climes may be related to heavy rainy seasons. Waterborne diseases can spread more after rainfall, such as cholera. Mosquitoes may multiply after rain; vectorborne diseases, like malaria, dengue, and chikungunya, can increase after rain.
The risks for disease change a bit as the seasons change. It's always a good idea to avoid disease risks.