The virus itself can cause varying symptoms within carriers; ranging from effects comparable to the annual seasonal flu, to more severe sickness; muscle pain and heavy vomiting. People with underlying health conditions, respiratory conditions, diabetes and heart conditions, as well as those who are pregnant are the most likely to suffer severe complications or to die as a result of infection.In its severest form, the virus attacks deep in the lungs and creates more severe respiratory symptoms than other flu viruses, which attack only the upper respiratory system. This can cause more serious breathing difficulty than other influenza viruses and increasing the risk respiratory failure.Swine flu carries a higher mortality rate than the seasonal flu, but the difference is greatly exaggerated by most tabloid newspapers. With the winter approaching in the northern hemisphere, however, the number of cases may take a long time to plateau.So why has swine flu become such a serious health topic? One reason may be its prodigious rate of spread. With air travel and international travel now commonplace, the rate of viral spread is increasing. SARS and Bird Flu are perfect examples of recent viral outbreaks where air travel was a major factor in the rapid spread of disease.Alarming virus epidemics have occurred in the past. In 1918, Spanish flu was spread to sub-Saharan Africa by two soldiers returning home from World War I, and caused the deaths of approximately two million Africans. The Black Death of the 1340s is another example of a lethal and easily transmitted infection, which claimed at least 75 million lives.Now with air travel widespread, disease can travel internationally to numerous countries, creating silent epidemics where the virus has spread widely before it can be controlled. Border control has also been stepped up by several countries such as China in a bid to curb the spread.Swine flu is an excellent example of how modern life can make it easier for diseases to spread at pace. The study of its epidemiology may provide vital data to scientists that could be used to prevent other diseases from spreading in the future, and thus ensuring that minimal lives are lost.In the meantime, frequently washing your hands and covering your mouth when coughing or sneezing is the most effective way of preventing the spread of disease, as eighty per cent of all bacteria are transferred via hand contact.