Respiratory illnesses decline in summer because high temperatures, increased ultraviolet (UV) radiation, and lower indoor crowding reduce the stability and transmission of viruses. According to Santé publique France, influenza activity typically vanishes during the summer months, with the 2023-2024 season ending in February and remaining undetectable through the warmer period.
Why do we get fewer colds and flu in the summer?
Environmental factors act as a natural disinfectant. High temperatures and UV rays damage the lipid envelope of many respiratory viruses, making them less stable and shorter-lived outside the human body. Humidity also plays a role; while very dry winter air can dehydrate nasal membranes, moderate summer humidity helps maintain the protective mucus barrier in the respiratory tract.
Behavioral changes further drop the infection rate. People spend more time outdoors in well-ventilated spaces. This prevents the concentration of viral droplets that occurs in closed, heated rooms during winter, which is the primary driver of seasonal spikes in rhinovirus and influenza.
Can you still get the flu during July or August?
It’s rare but possible. Most “summer flu” cases are actually other viral infections, such as adenoviruses or enteroviruses, which cause fever and fatigue but aren’t influenza. True influenza cases in the summer usually occur in travelers returning from the Southern Hemisphere, where winter peaks between June and August.
Which health risks increase during the summer?
While respiratory viruses dip, other pathogens thrive in the heat. Bacterial growth accelerates when the “cold chain” is broken, leading to a rise in foodborne illnesses. Salmonella is a primary culprit, often found in undercooked poultry or improperly stored foods.
Enteroviruses also peak from May to October, with a maximum surge in July and August. These viruses are highly resistant to heat and spread easily in crowded summer environments like campsites, summer camps, and swimming pools. Common manifestations include:
- Hand-foot-and-mouth disease: Common in young children via direct contact.
- Viral meningitis: A potential complication of enteroviral infections.
- Gastroenteritis: Spread through the fecal-oral route or contaminated water.
Summer-Specific Infections
| Condition | Primary Cause | Common Trigger |
|---|---|---|
| Otitis Externa | Bacteria in moist canals | Swimming (“Swimmer’s Ear”) |
| Urinary Tract Infections | Bacterial growth/Concentrated urine | Dehydration from heat |
| Folliculitis | Aquatic bacteria | Contaminated pools or lakes |
| Vector-borne diseases | Ticks and Mosquitoes | Lyme disease, Dengue, Chikungunya |
What happens to virus levels when school starts?
Epidemiologists observe a mechanical surge in infections following the return to school in September. This isn’t necessarily because the viruses “wake up,” but because children move from open-air environments back into closed, poorly ventilated classrooms. This proximity facilitates the rapid spread of droplets, leading to a spike in laryngitis, the common cold, and other upper respiratory tract infections.
Preventing Summer Illnesses
To avoid seasonal pitfalls, health officials recommend specific precautions based on the risk factor:
- Food Safety: Keep perishable foods refrigerated and cook meats to safe internal temperatures to prevent Salmonella.
- Hydration: Drink ample water to maintain urinary volume and reduce the risk of UTIs.
- Water Hygiene: Shower after swimming in lakes or stagnant water to prevent skin infections and folliculitis.
- Vector Protection: Use insect repellent in wooded or tropical areas to avoid tick-borne and mosquito-borne pathogens.