Seasonal diseases: why infections have a calendar and why it differs by country

When people say "seasonal diseases" they usually mean winter colds and flu. One review of infectious diseases suggests that is a small corner of the picture: it collected more than 60 infections of public-health importance and found that many of them have a time of year when they rise. Cholera, chickenpox, polio, dengue and meningococcal infection are all in it, and their seasons do not agree with one another.

This article is a map of that calendar, not a guide to any one illness. It is informational material, not medical advice. If symptoms are severe, getting worse, or not going away, that is a question for a doctor.

Seasonal diseases are more than flu

The source is a 2018 review in PLoS Pathogens, "The calendar of epidemics". It covers more than 60 infectious diseases of public-health relevance, and its tables run to roughly 69 rows. It groups the causes of seasonality into four kinds: the environment, the behavior of the host, the biology of the host through the year, and other living things around the host (Martinez, 2018).

So "seasonal" does not mean "winter" and it does not mean "cold". A disease can follow the rains, the school year or the calendar of farm work. Which season it picks depends on what the disease needs in order to spread, and that differs from one infection to the next.

Why some diseases are seasonal: eight drivers

The review names eight specific drivers:

1. seasonality of vectors, the animals such as insects that carry the infection;
2. seasonality of animal hosts;
3. seasonal climate;
4. non-climatic non-living environment;
5. seasonal co-infection, where one infection changes the odds of another;
6. seasonal contacts and behavior;
7. the seasonal living environment;
8. seasonal exacerbations or remission in a person who already has the disease.

Here are the examples the review gives, as it states them. This article does not match each one to a single driver.

• Cholera: outbreaks usually in the warm months, and the seasonality is stronger in countries farther from the equator.
• Rotavirus: peaks in December and January in the southwest of the United States and in April and May in the northeast.
• Chickenpox: a spring peak in both the Northern and the Southern Hemisphere.
• Polio: epidemics in summer, in the United States.
• Measles: linked to children gathering in schools, and in developing countries to agricultural cycles.
• Dengue: linked to the rainy season, in Thailand.
• Meningococcal infection: rising in the dry season in sub-Saharan Africa.

Taken together, they show there is no single "bad season". Summer polio, spring chickenpox and dry-season meningococcal infection sit on the same calendar as winter respiratory viruses.

One disease, different seasons in different places

The more surprising result is that the same disease can run on different calendars depending on where it is.

Rotavirus is already one example above: December and January in one part of the United States, April and May in another. Hepatitis C is another. The review reports spring or summer peaks in Egypt, China and Mexico, and winter in India (Martinez, 2018).

That is why a statement such as "disease X peaks in month Y" is only safe with a place attached to it. It also explains why articles about "the" season of an infection tend to disagree. They are often describing different places.

Respiratory viruses: the case with the most site data

For flu and a few related viruses there is far more location data than for most infections, so they make the clearest example of how the calendar shifts with geography.

A systematic analysis of 246 sites for influenza, 183 for respiratory syncytial virus (RSV), 83 for parainfluenza and 65 for metapneumovirus found that influenza had clear winter epidemics at most temperate sites. The closer a site was to the equator, the less predictable and less seasonal the timing. The epidemic lasted 3.8 months (3.6 to 4.0) at temperate sites and 5.2 months (4.9 to 5.5) in the tropics. RSV behaved differently: it was seasonal in both the temperate zone and the tropics, starting in late summer in the tropics of each hemisphere and reaching most temperate sites in winter. Parainfluenza was found mostly in spring and early summer in each hemisphere (Li et al., 2019).

A review of 137 locations found the same broad split: flu and RSV consistently peaked in winter in temperate places and varied more in the tropics. Some temperate places had two flu peaks a year, in winter and in summer, and so did tropical Southeast Asia more often. Of the tropical locations, 80% had a distinct RSV season of 6 months or less, and 50% a distinct flu season of that length (Bloom-Feshbach et al., 2013).

In the United States, the CDC puts it plainly: "In the United States, flu season usually occurs in the fall and winter", and "most of the time flu activity peaks between December and February" (CDC). In the Southern Hemisphere winter falls in June to August, so a temperate-zone winter peak lands in different months there. The details by hemisphere and by climate are in the flu season by country. For why cold itself is only part of the story for winter colds, see why we get sick more in winter.

What nobody knows yet

• Which driver explains which disease in which place. The examples above say when and where each infection rises. This article does not claim more about the cause than the review does.
• What happens where nobody has counted. Bloom-Feshbach and colleagues say seasonal data for flu and RSV remain limited in large undersampled regions, including Africa and Central America. Whatever the real calendar is there, it is not in these sources.
• What an average calendar says about this year. The CDC says "usually" and "most of the time"; a particular season can start earlier or later than the typical one.
• Whether seasons for the same disease stay put. The sources are descriptions from past data in specific places, not forecasts.
• Whether a season means the same thing for you. These are population patterns. They do not tell you your own risk in a given month.

Seasonality of infections also should not be confused with seasonality of deaths. How winter mortality varies between countries is a separate question, covered in excess winter deaths.

What you can look at in your own data

The app records local outdoor weather (pressure, temperature, relative humidity, wind speed, UV index) next to your own symptom entries and Apple Health data, and looks for correlations in your own history with Pearson correlation, ANOVA and false-discovery-rate correction. Its nine fixed symptoms are headache, fatigue, drowsiness, dizziness, anxiety, irritability, nausea, joint pain and insomnia.

What it does not do matters more here. It does not recognize, diagnose or predict infections, and cough and fever are not among its nine symptoms. A seasonal infection is outside what the app can see. What it can show is whether the symptoms you do log line up with outdoor weather in your own entries. Why such a pattern needs care before it is called a cause is covered in weather sensitivity: what the science says.