Spikes in deaths from heart disease have been recorded after major earthquakes and other natural disasters. (Fotolia.com / May 8, 2013)

The terrorist bombings and subsequent manhunt in Boston last month left four innocent people dead and many more injured. But the stress caused by these tragic events could adversely affect the health of a much wider population.

The citywide shutdown, the unprecedented use of social media to discuss unfolding events, together with the extreme anxiety generated by the bombings could all contribute to a higher toll on health.

We know large-scale attacks and natural disasters claim more victims than those directly injured. For example, Robert Kloner, a professor or cardiovascular medicine at the University of Southern California-Los Angeles and Director of Cardiac Research at Good Samaritan Hospital-Los Angeles, studied the effects of the Northridge earthquake, which struck Los Angeles in 1994, killing 57 people. Deaths from heart disease in the area jumped from 73 per day before the earthquake to 125 on the day of the disaster.

Similar spikes have been recorded after other major earthquakes, and the Iraqi missile attacks on Israel in 1991. Most victims probably had underlying heart disease.

When Kloner and his colleagues carried out a similar study in New York City after the 9/11 terrorist attacks, they were surprised to find no extra spike in heart deaths. Kloner thinks that you may have to fear for your life before stress can trigger a fatal attack. In New York, he suggests most of those people would have been in the twin towers and so died in the attack itself.

But widespread fear and stress caused by disasters and terrorist attacks can exacerbate health problems. A study in Japan followed people with hypertension who were already recording their heart rate and blood pressure each morning when the earthquake and tsunami struck in March 2011. On the day of the disaster, there was a dangerous rise in both blood pressure and heart rate that took several weeks to return to normal.

After 9/11, Manhattan residents reported worsening asthma symptoms even if they weren't directly involved in events. And Jonathan Steinberg, a cardiologist at Columbia University in New York, found that in the month following the tragedy, the number of potentially fatal heart arrhythmias in the area more than doubled. A similar effect occurred in Florida, suggesting that simply watching events on television can affect people's hearts.

Indeed, another study found that the number of cardiovascular ailments across the U.S. rose by more than 50 percent in the three years following 9/11.

Many types of stress can be detrimental to your health, but situations like the Boston bombings -- when large numbers of people face the same event at the same time -- provide a rare opportunity to study, for example, how acute stress can cause heart failure. On top of chronic risk factors such as obesity, "we learn what the triggers are," says Kloner.