There are three main types of bird flu viruses, and not all of them pose a threat to humans. Influenza A viruses are the types that all infect and are carried by birds, and within each group several subtypes exist; all subtypes occur in birds, but only some can cross into humans. Most commonly found in humans is the hemagglutinin 9 (H9) type. Hemagglutinin 5 (H5) has subtypes that can be deadly to humans but are less common, while human infection from hemagglutinin 7 (H7) typically occurs only when an individual comes in contact with an ill bird, instead of spreading from person to person.
The subtypes of bird flu viruses are defined by a second letter and number combination after the first set. The first number defines the number of hemagglutinin proteins contained in the virus, while the second number defines the amount of neuraminidase (N) proteins. Most of the H5 types of bird flu viruses have numerous different mutations that allow for a number of different N numbers.
One of the most potent of the bird flu viruses that has crossed over into humans is the H5N1 strain. This type of avian flu has had a relatively high mortality rate among humans from the time it first infected individuals in Hong Kong in 1997. The virus causes typical flu symptoms such as a fever, body aches, and an overall feeling of fatigue and weakness. In some cases, it has been known to culminate in organ and respiratory failure. The development of this virus in humans has shown how the flu can mutate to spread through entirely different types of creatures.
Influenza A viruses are so named because every type and subtype is found in birds. Some subtypes can be deadly to birds, especially domestic ones. Others exist as a mild illness and only pose a threat to another species once mutated. All of the H5 subtypes have been documented to occur in humans, while H7 and H9 subtypes occur rarely in humans and are generally mild.
When occurring in birds, there are two main outcomes of a bird flu outbreak. Some of the viruses result in only an unhealthy look to the feathers and a change in the number and quality of eggs, but can be deadly when mutated to spread to humans. The other type, known as high pathogenic, is considerably more deadly to birds. Once a species is infected with a subtype of one of the bird flu viruses, the disease typically spreads quickly.