Someone sick with Ebola might not be able to provide a comprehensive list of everyone they've come in contact with.
Isolation zones have been set up in Mbandaka's main hospital as well as Bikoro hospital, with more Ebola treatment centers being set up in Mbandaka and Bikoro.
The scale of the previous outbreak forced several nations, including Russian Federation and China, to develop vaccines to Ebola.
The disease has spread from a rural area of the country's Equateur Province where it was first reported last week to the city of Mbandaka, which has a population of more than 1,000,000 and is on the banks of the Congo River which connects several countries. "If we don't, then the virus may spread too far and too quickly".
"This is a major development in the outbreak", said Peter Salama, the WHO's deputy director-general of emergency preparedness and response.
"We're certainly not trying to cause any panic in the national or global community", Salama said.
A ministry statement on Thursday said that the total number of Ebola cases is now 45, including 10 suspected, 21 probable and 14 confirmed.
Doctors Without Borders (MSF), quoting local officials, said 514 people were believed to have been in contact with Ebola patients and were now being monitored.
"Since the announcement of the alert in Mbandaka, our epidemiologists are working in the field with community relays to identify people who have been in contact with suspected cases", Ilunga said.
Ebola is lethal and highly contagious, which makes it hard to contain - especially in urban environments where people are mobile and come into more contact with others.
Previously the disease was suspected in patients only in rural parts of the country, but reaching the city of Mbandaka makes the outbreak, which was first reported on May 8, significantly bigger than previously thought. The World Health Organization (WHO) deployed 4,000 dozes to Congo to help fight the latest outbreak.
The latest Ebola case in the Democratic Republic of Congo comes amid an outbreak of violence that has spread rapidly in the country's restive Kasai region.
They are coordinating their efforts - and that includes not only tracking down and monitoring contacts of infected people, but a plan to give those contacts the experimental vaccine.
There is no licensed drug to treat or prevent Ebola, although an experimental vaccine arrived in the DRC on Wednesday and has been cleared for use by the Kinshasa government.
He said the disease could have been taken there by people who attended the funeral of an Ebola victim in Bikoro, the south of Mbandaka, before travelling to the city.
The vaccine, from pharmaceutical firm Merck, is unlicensed but was effective in limited trials during the West Africa outbreak.
More than 11,000 people died in an Ebola outbreak that plagued the western African countries of Liberia, Sierra Leone and Guinea for about 2 years from 2014. It involves vaccinating contacts of those feared infected, contacts of those contacts, and health care and other front-line workers.