Tuesday August 5, 2014 8:24 AM UPDATED: Tuesday August 5, 2014 8:35 JOHANNESBURG, South Africa - The worst outbreak of the deadly Ebola virus in history could actually be much worse than the official death toll reflects. Already, the World Health Organization says 887 people have died, but a top doctor working at the heart of the outbreak in West Africa says many cases are going unreported. The senior doctor, who works for a leading medical organization in Liberia, explained to CBS News' Debora Patta that what has helped set this outbreak apart from previous ones is the virus' spread in urban areas. One of the epicenters of the disease is the Liberian capital of Monrovia, home to about a million people, or almost a quarter of the country's population. The doctor, who spoke to CBS News on condition of confidentiality, said the disease is spinning out of control in Africa partly because it is extremely difficult to contain it in a sprawling, congested city center.