Though it's known for its debilitating symptoms - think coughing, fever, sneezing, muscle fatigue, nausea and more - just 23 percent of those infected with the flu presented the telltale signs. That's according to a recent study by British scientists that surveyed the flu diagnosis and infection rates over five years, observing more than 5,000 people.

Those surveyed suffered from both seasonal flu and H1N1, or 'swine' flu, which emerged in 2009. The data was pulled from flu seasons in the years 2006-2011, with around 18 percent of participants becoming infected with influenza. However, less than a quarter of those who contracted the illness displayed common symptoms, and only 17 percent actually visited a primary care doctor for treatment. Another unexpected pattern that emerged was the severity of the H1N1 strain, which in many cases had its symptoms outweighed by those of seasonal iterations of the disease - meaning that the pandemic version of the flu was not outwardly more threatening than the more common winter flu.

"Reported cases of influenza represent the tip of a large clinical and subclinical iceberg that is mainly invisible to national surveillance systems that only record cases seeking medical attention," said the study's lead author Dr. Andrew Hayward of University College London in a press release.

"Most people don't go to the doctor when they have flu," he continued. "Even when they do consult they are often not recognized as having influenza. Surveillance based on patients who consult greatly underestimates the number of community cases, which in turn can lead to overestimates of the proportion of cases who end up in hospital or die."

The results of the study are indicative of how few people actually refer to doctors for treatment, meaning that studies based solely on data gleaned from doctor's visits provide insufficient information for ongoing research around the flu. Further, the information additionally suggests that even sufferers without obvious symptoms play a substantial role in the spread of the flu.

The study, titled Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study, was published in The Lancet Respiratory Medicine Journal on March 17 2014.