There are reasons to be concerned about H7N9, the new strain of bird flu discovered last month in China, but there are also reasons to be reassured.

The good news is that the new strain doesn’t appear to be able to pass from person to person. That’s clear in the fact that there have been only 100 or so cases documented so far, all confined to China. Infectious diseases expert Dr. Neil Rau says if this new virus were as easily transmissible as the H1N1 outbreak of 2009 for example, the number of infected people would be “galloping,” with new cases popping up all over the regions.

“The fact that this has been going on for at least a few months with only this amount of cases, to me, is reassuring. If this were a really serious problem, this strain, we would have many more cases by now,” he recently told CTV News Channel.

Nevertheless, the World Health Organization says there is reason to be vigilant about this new strain. Here are five reasons why:

1. It’s never been seen in humans before

Bird flu strains usually don’t infect humans. And when they do, they might cause red eyes or throat soreness; this strain is causing a severe and often fatal form of pneumonia.

Flu experts have long warned that the next flu pandemic would likely come from a new strain to which most humans would have little or no immunity – most likely from another animal species.

The World Health Organization says the new human cases of H7N9 are "of concern" because they are completely new. There have been other H7 viruses that have infected people, but never an H7 and an N9, which are the names given to the proteins of the virus' surface.

"That makes it a unique event, which the World Health Organization is taking seriously," the health agency said in mid-April.

2. Virus has learned to adapt to mammals

The Chinese Centre for Disease Control and Prevention has sequenced the genes of H7N9 and its experts say it’s clear the strain began in birds. But the virus also showed several genetic changes that suggest it is “learning” how to attach to the airways of mammals and humans, which would allow it to spread easily.

One of the specialists who has studied the virus’ genes, Japan’s Dr. Masato Tashiro, says while the virus still not yet adapted to humans completely, “important factors have already changed."

That has the World Health Organization worried, because flu viruses are constantly mutating and swapping genes.

Where there have been only about 100 cases in a country of 1 billion, the cases are coming quickly. H7N9 has already caused more human infections in a single month than H5N1 bird flu does in a single year.

"When we look at influenza viruses, this is an unusually dangerous virus for humans," the WHO flu expert Dr. Keiji Fukuda told a news briefing in late April.

Fukuda says the virus carries some genetic mutations that suggest it is better adapted to infecting mammals than regular bird flu viruses, which is why experts like him are so concerned.

3. The original source of the infection still isn’t clear

So far, it appears that live poultry markets where live ducks and chickens are sold are where most of the human infections derive. Samples from chickens, ducks and pigeons from poultry markets have tested positive for the virus, while those from migratory birds have not.

Another clue that seems to support this is that on April 6, officials in Shanghai (which has seen most of the deaths in this outbreak) closed all poultry markets in the city and ever since, there has been a "dramatic slowdown” of new cases in the city.

As well, a large proportion of those infected have been med over the age of 65. That could mean that these men are more susceptible to the virus than others, or it could be a reflection of the fact that men tend to be the ones who shop for poultry at live bird markets.

But perhaps live markets aren’t the only source of these infections. A number of those who have become ill have reported that they had no exposure to birds. In fact, some reports suggest that anywhere between 25 and 60 per cent of those infected had no bird contact. More investigation is needed to better understand this.

It’s also unclear how the virus is making its way to the markets in the first place. And it’s still isn’t clear if the birds are picking up the virus from another species –- perhaps pigs -- that could also be leading to human infections.

4. The virus appears to cause no symptoms in birds and possibly pigs

The scientists who inspected the genes of the virus samples sent to Chinese labs say H7N9 appears capable of infecting some birds without causing any symptoms. That means it could be a "silent spreader" that’s moving among birds or pigs undetected.

Without obvious outbreaks of flu in chickens, wild birds or pigs, that could make it difficult to pinpoint the original infection source, as well as track its spread. That wasn’t the case with H5N1 bird flu, which caused obvious signs of illness during outbreaks, allowing farmers to perform mass culls to stop the virus’ spread.

It also makes it harder to narrow down the species that’s fuelling the virus’ spread.

"At the moment, we can't see where this virus is coming from. We don't know yet what animal source is feeding this." Wendy Barclay, a flu virology expert at Britain's Imperial College London recently told Reuters.

And, if there were no obvious symptoms in birds or pigs "nobody recognizes the infection in animals around them. Then the transmission from animal to human may occur," Tashiro told Reuters.

"In terms of this phenomenon, it's more problematic."

5. Credibility of Chinese officials

Finally, there are concerns that flu experts may not be getting the complete picture about this new virus.

In 2003, Chinese authorities initially tried to cover up the emergence of a baffling condition that became known as SARS. Chinese officials later had to apologize for their slow response and promised to be more forthright during future pandemics.

China’s Health Ministry says the government is committed to communicate details of the new strain to the outside world and its own people. The Chinese Centre for Disease Control and Prevention has already shared the genetic sequence of H7N9 with other scientists around the world.

But because two of the victims fell ill in February, some are questioning why it took so long for the government to announce the new cases.

The WHO so far has praised the Chinese government, saying it’s responding well, offering detailed case management and tracing contacts of all those known to have been infected so far.

Nevertheless, it’s likely that worries will continue that given China’s track record, authorities there may not be passing on all that they know.

With files from The Canadian Press and the Associated Press