Experts warn the disease may have spread as people criss-crossed the country in packed buses during the festive season.
At least 44 people have died from Lassa fever in Nigeria with the death toll expected to rise, underscoring the difficulty in combating deadly viruses in a region still reeling from Ebola.
While health authorities assure Africa's most populous country of more than 170 million they have the haemorrhagic virus under control, there are fears the scale of the outbreak is being downplayed.
The outbreak was only announced in January -- months after the first case of the disease happened in August -- with subsequent deaths reported in 10 states, including in the nation's central capital Abuja and the bustling oil hub of Port Harcourt.
Experts warn the disease, transmitted through the faeces, urine and blood of rats as well as human bodily fluids, may have spread as people criss-crossed the country in packed buses during the festive season.
"It is possible we are only seeing the tip of the iceberg," Chikwe Ihekweazu, an infectious disease epidemiologist, told AFP.
"My biggest worry at the moment is that given that the cases that have been reported are from a very wide geographical area, transmission chains will be difficult to identify, making control very difficult."
During the massive Ebola outbreak in 2014, the World Health Organization (WHO) described Nigeria as a "spectacular success" story, praising Africa's biggest economy for preventing an "apocalyptic urban outbreak" with strong leadership and a rapid response.
Only seven people died from 19 confirmed cases of the disease out of more than 11,000 deaths in west Africa.
Yet the recent Lassa flare-up indicates Nigeria's health care system is not so resilient, with issues ranging from basic health education to doctor diligence.
Michael Asuzu, professor of public health at the University of Ibadan, in southwest Nigeria, said the Lassa response took so long to gear up because residents in the initial infected village of Foka, in the northwest state of Niger, attributed deaths to supernatural forces.
"They thought it was related to some traditional things they were doing at the time, so there was cultural secrecy surrounding it," said Asuzu, who is chairing an expert committee appointed by the government to investigate the outbreak.
"They attributed it to God's will."
Scientists familiar with the disease say that though Lassa fever outbreaks are not uncommon in Nigeria, the death toll of the recent outbreak is alarming.
"From the numbers that are reported, it think it's really bad for the country," Lassa researcher Deborah Ehichioya said. "It's not normal."
Doctors could be misdiagnosing Lassa fever, facilitating the spread of the disease, she added.
"It's not just business as usual, which is treating for malaria and typhoid, we have a lot more to diagnose," she said.
Lassa fever is an acute haemorrhagic illness which belongs to the arenavirus family of viruses, which also includes the Ebola-like Marburg virus, according to the WHO.
The virus is endemic in rodents in west Africa and is usually transmitted to humans by contact with food or household items contaminated with the animals' faeces and urine.
The number of Lassa fever infections in west Africa every year is between 100,000 to 300,000, with about 5,000 deaths, according to the US Centers for Disease Control and Prevention.