A deadly contagious virus can race around the world at a ferocious speed killing millions of people.
Diseases such as lethal forms of flu, smallpox, SARS and HIV.
Viruses are the ultimate parasite.
Dr Andrew Simor, Sunnybrook Hospital, Toronto - "Viruses are really like the Trojan horse, because they inject themselves into human cells and use DNA in order to replicate and multiply and then spread to the next cell."
When a virus enters our body it penetrates a cell.
Virus penetrates cell and it takes over the cell's own materials and uses these to reproduce.
Once the cell is overloaded the virus particles break out, attacking other cells throughout your body, spreading infection.
Factors which fuel the fear of global infection include rapid urbanization, environmental mismanagement and intensive farming practices.
Infection risk factors:
Which all make human populations more vulnerable.
And modern travel and global trade means that once contamination has occurred, international spread is more likely.
Pandemic spread through:
Some fear that pandemic viruses could even be propagated intentionally as a form of weaponry, also called biological warfare or bioterrorism.
Other potential risks:
Biological warfare or Bioterrorism
However, the successful eradication of pandemic viruses, such as smallpox, gives hope that medical advances can protect us from the next global outbreak.