As the Covid-19 coronavirus spreads increasingly across the globe, "the threat of a pandemic has become very real" and New Zealand is now not only testing those with concerning travel history and typical symptoms, but also those hospitalised with other serious respiratory illnesses.

Photo: Kena Betancur / Getty Images / AFP

Here is the difference between a suspected case and a probable case, as well as the case definitions set by the Ministry of Health.

Director-General of Health Dr Ashley Bloomfield said a suspected case "is someone who fits the criteria of travel exposure or exposure to a confirmed case and has symptoms consistent with Covid-19 before they are tested".

If the testing is positive, then they become a confirmed case.

If the testing is negative - but there's a strong clinical suspicion based on the travel history and symptoms - then it becomes a probable case.

"The professional consensus is that this person has had Covid-19 but the laboratory testing didn't confirm it," he said.

He said research suggested there was a very small possibility that a person in the same row on an airplane as that of an infected person would contract the virus.

"Modern aircraft have fantastic filtration and ventilation systems."

Of the five cases in New Zealand, anyone who was a close contact, or who was symptomatic when in contact, will have been tested.

"So far we've had only negative tests over the last two days," Bloomfield said.

He added that from last week, anyone in ICU (intensive care unit ) who had a severe respiratory infection was being tested for Covid-19.

"This is part of what we call surveillance."

Furthermore, as influenza season approached, he said, swabs that were taken for influenza type A or B would also be tested for Covid-19.

"That gives us a really good mechanism as we head into winter to find out is there Covid-19 out there in the community."

Read more about the Covid-19 coronavirus:

According to the Ministry of Health's case definition for Covid-19, a suspected case must meet both the epidemiological and the clinical criteria.

A suspected case can be defined as either of the following:

"Fever or history of fever (≥38C) and acute respiratory infection with either shortness of breath, cough or sore throat., in addition to travel to or from (excluding airport transit) countries or areas of concern within 14 days before the onset of illness.

"Any acute respiratory illness and close contact with a suspect, probable or confirmed case of SARS-CoV-2 infection in the 14 days before onset of illness."

For surveillance purposes, the ministry will test and label it a suspected case if a person has "severe acute respiratory illness (fever and at least one sign/symptom of respiratory disease like cough, shortness of breath) and is hospitalised in addition to no other aetiology that fully explains the clinical presentation and no source of exposure has been identified (ie, regardless of travel history)".

The ministry's other definitions include:

Under investigation: A case that has been notified, but information is not yet available to classify it as suspect, probable or confirmed.

Suspected: The patient is classified as a suspected case, pending further investigation, if they satisfy both the clinical and epidemiological criteria.

Probable: A case that meets both clinical and epidemiological criteria where other known aetiologies that fully explain the clinical presentation have been excluded, and either has laboratory suggestive evidence or for whom testing for SARS-CoV-2 is inconclusive.

Confirmed: A case that has laboratory definitive evidence.

Not a case: A case that has been investigated and subsequently found not to meet either the probable or confirmed case definition.