Exercise is healthy. But it can also be harmful, with fatal consequences, if the heart muscle becomes inflamed. It can lead to cardiac arrest. And it's more common than you might think.
Ursula Hildebrandt's "enemies" are small - so small, in fact, that they can only be seen with a microscope. But they are deadly.
Her enemies are pathogens that enter the heart through the bloodstream and lay there in wait - with potentially fatal consequences.
Hildebrandt is a 31 year old physician at the German Sport University in Cologne, where she treats athletes suffering from myocarditis. It's a disease that attacks the "motor" in all of us - the heart muscle that pumps blood through the body.
If inflamed, the heart muscle becomes weaker and its capacity to pump blood declines, and the body stops receiving its vital supply of blood.
"Anyone can develop myocarditis - athletes as well as couch potatoes," says Dr Hildebrant.
Susceptible to infections
But athletes and physically active people are particularly at risk. Myocarditis, a heart muscle inflammation, can occur when an athlete resumes training too soon after a viral infection or a common cold.
Sports can become dangerous when we are physically weak. Our bodies become more susceptible to infections after exercise. And when bacteria or virus manages to penetrate the body, they basically tumble around in the blood.
When we exercise, blood flows faster through the four cavities of the heart and the valves that control the flow of blood through the various sections. The blood swirls around the heart's valves, like a river with a strong current that flows through a lock and creates a whirlpool. And when a person jogs or plays soccer, blood gushes through the heart even faster. But when it does, pathogens can get stuck behind the heart valves and cause an inflammation.
Hildebrandt says the best protection against myocarditis is for us to recover thoroughly from any illness before we exercise.
"If you've had a fever, the general rule of thumb is for you to wait at least two days after it's gone down and you've stopped presenting green or yellow phlegm," says Hildebrandt. Phlegm indicates that dangerous germs are still moving around the body.
One of the problems that Dr Hildebrandt faces in fighting pathogens is that they have a variety of symptoms. They range from severe acute symptoms, such as chest pain or shortness of breath, to creeping indications such as a high resting heart rate or night sweats.
This is also one of the reasons why myocarditis is often diagnosed late.
If people overexert themselves while ill, an undetected myocarditis can lead to cardiac arrest and death.
In 2009, the German track athlete René Herms was found dead in his apartment. The autopsy showed that the 26 year old had died of an undetected myocarditis. He had worked out before he died.
The same could have happened to Fabian Spinrath - an athlete, who has won many medals. It all began after the 800 meter runner felt unusually tired after a long run.
"Normally, you rest for about 10 minutes afterwards," says Spinrath. "I slept for five hours."
His physician suspected a bacterial infection and prescribed antibiotics. The symptoms remained unchanged for six weeks and it was only after a further examination that Spinrath was diagnosed with myocarditis.
The diagnosis was a huge physical challenge for the 18 year old athlete. For six months, he had to avoid any physical exertion. He took the elevator instead of the stairs. He wouldn't even run a couple of meters to catch a streetcar. Training was out of the question.
After six months, Spinrath began to work out again, and he has now recovered from his bout with myocarditis. His heart functions as it did before the inflammation.
"We wanted to avoid any permanent damage," says Hildebrandt, who is treating the young athlete. "If we had let him start any sooner, his heart function could have remained constricted for the rest of his life."
Healthy diet and plenty of sleep
Each myocarditis is different and requires an appropriate therapy. But it is advised that patients should avoid any physical exertion for at least six months. They should also avoid alcohol, maintain a healthy diet and sleep as much as possible. Medication can also be useful.
Myocarditis is also found in developing countries. But the pathogens in these countries are mostly bacteria and fungi, while viral infections are the main cause of myocarditis in developed countries.
Ursula Hildebrandt's "enemies" may be microscopic and take different forms, but they all have one thing in common - they can be prevented if people are willing to take care of themselves with every cold and infection and listen to what their bodies tell them.