Until recently, however, humans rarely became infected with this type of virus.
Geological service researchers have blamed a type of virus called an iridovirus for most of the deaths in North Dakota.
Retroviruses are a type of virus that can also cause mutations in the adult cells, making them cancerous.
The type of virus thought to cause SARS, a coronavirus, has never received serious attention from drug makers.
The trick begins with a type of virus called a bacteriophage.
The type of virus used as a gene carrier has widely been considered safe and is being used in 35 other trials.
Another caveat is that, so far, scientists use a type of virus, a retrovirus, to insert the genes into the cells' chromosomes.
The hemophilia treatment uses an adeno-associated virus, a different type of virus to ferry the genes into the body from the technique in those two cases.
Ms. Parker said the company had used the same type of virus to carry various genes into more than 500 patients and had not seen similar problems.
Flu viruses mutate a lot, and pandemics occur when a Type A virus undergoes a big genetic change to produce a new type of virus to which people have little or no resistance.
Then, after searching published reports on known drugs, they landed on the Pfizer drug, which works in a very similar way to block a protease in another type of virus, a rhinovirus, which causes many colds.
