Still, there is some good news on the polio front.
Sunday marks one year since Type 3 polio viruses have been found, suggesting vaccination efforts may — heavy stress on may — have wiped out the second of three strains of polio.
If the Type 3 viruses are indeed gone, it will mean that only Type 1 polio viruses remain to be vanquished before the long-overdue goal of polio eradication can be realized.
Reaching the one-year milestone has made people involved in the frustrating and expensive polio eradication program hopeful that transmission of Type 3 viruses may have stopped. But they warn it would be unwise to declare victory just yet.
"I think the available information is cause for some cautious optimism. But in particular in Nigeria I think we would look for more evidence before we would ... uncork the champagne," says Olen Kew, a senior virologist specializing in polio at the U.S. Centers for Disease Control in Atlanta.
The most recent Type 3 viruses seen were spotted in Nigeria in November 2012. Pakistan, another of polio's strongholds, last reported a Type 3 infection in April of 2012. And the CDC's polio laboratory, which monitors movement of viruses worldwide, had been reporting that the number of lineages of Type 3 viruses — families within the overall group — has been markedly declining in the past couple of years.
But in both Nigeria and Pakistan, the most recent Type 3 cases were found in politically troubled parts of the countries where vaccination teams struggle for access to children. In Nigeria, the experts know, surveillance for the virus is not as strong as it might be — hence Kew's comment urging caution.
In Pakistan, however, environmental surveillance — testing sewage for polio viruses — makes the situation look more promising.
The last Type 3 virus there was seen in the FATA, the federally-administered tribal areas bordering Afghanistan where polio vaccinators have often been refused entry for long periods. The nearest site where environmental testing is done is Peshawar, where last month a bomb was detonated while polio vaccinators collected their kits.
"The viruses from the tribal areas find their way into Peshawar (sewage) regularly," says Kew. "And the fact that we've not seeing it in the environment in Peshawar is really quite encouraging."
The CDC is one of founding partners in the polio eradication project, which started in 1988. The World Health Organization, UNICEF and the service club Rotary International are the other founding partners; in more recent years the Bill and Melinda Gates Foundation has become a key player in the effort.
Buoyed by the success of the smallpox eradication program — the disfiguring disease was declared eradicated in 1979 — and by the ease with which the Americas stopped polio transmission, the world embarked on the plan to get rid of polio by the year 2000.
But this task is vastly more complicated, because of the nature of polio.
Whereas people who were infected with smallpox were easy to spot — they developed the scarring rash — much of polio's spread is invisible to the eye. Only a small portion of cases develop paralytic polio. And where there was one type of smallpox, there were actually three strains or serotypes of polio.
"In a sense, polio eradication should be called polios eradication," says Dr. Walter Orenstein, who works on the eradication effort through the Emory Vaccine Center at Emory University in Atlanta.
"As opposed to smallpox, where we had to get rid of one agent, with polio we're really having to get rid of three agents."
One of those serotypes is actually long gone. Type 2 polio viruses haven't been spotted since 1999. And now it appears Type 3 viruses may be following Type 2 into the history books.
Each of the three viruses has unique qualities. Type 1 is the most aggressive and is able to spread long distances with ease. The viruses responsible for the Syrian outbreak as well as those being discovered in Israel sewage this year are all Type 1s from Pakistan.
Type 3 is a poor traveller. It has rarely been seen to jump long distances. But another quirk of the virus is making the experts watching this situation wary about claiming victory.
Of all the viruses, Type 3s are least likely to paralyze the children they infect. It is estimated that only one in 1,200 Type 3 infections leads to paralysis. By comparison, Type 1 viruses paralyze about one out of every 150 or 200 infected children.
There are only two ways to detect spread of polio viruses — by finding them in sewage, where environmental surveillance is done, and by finding paralyzed children. Type 3's low paralysis-to-infection rate makes it harder to spot.
"Because of the lower case-infection ratio, and because of the state of surveillance, (Type 3 can go) under the radar," says Dr. Steven Wassilak, a medical epidemiologist in CDC's global immunization division.
"And so when we hit the anniversary, particularly in an area" — Nigeria — "where the surveillance is less rigid than even in Pakistan, we have to be extremely cautious about what it means."
Still, the suggestion that Type 3 viruses may be gone is a welcome positive sign, coming in what has been a year of both record low cases and some extremely tough setbacks for the polio program.
"The prospects for completing eradication, they go up substantially with each serotype you knock out," says Dr. Bruce Aylward, the Canadian who has fronted the WHO's polio efforts for years.
"Mainly because of what you learn getting there.... And then of course there's that very real impact it has with governments," says Aylward, the WHO's assistant director-general for polio, emergencies and country collaboration.
The reference to governments is an allusion to the polio eradication program's constant struggle to raise funds. The effort has cost more than US$10 billion so far and it is estimated it will take another US$5.5 billion to complete the job.
Major funders have included Rotary International, the Gates Foundation and a number of governments, including Canada. Earlier this year the federal government pledged to contribute $250 million for polio eradication over the next six years.