Human influenza can be caused by: influenza A virus. influenza C virus. influenza B virus. influenza D virus.
The correct answer and explanation is:
The correct answer is influenza A virus, influenza B virus, and influenza C virus. Influenza D virus does not typically cause human influenza.
Influenza A virus is the most common cause of seasonal flu outbreaks in humans. It can infect a wide range of hosts, including humans, birds, and other animals. This virus is the one responsible for major flu pandemics due to its ability to undergo genetic changes. The virus can change its surface proteins (hemagglutinin and neuraminidase), leading to new strains that can evade immunity in the human population. Influenza A is further categorized into subtypes based on the variations in these surface proteins, such as H1N1, H3N2, etc.
Influenza B virus primarily affects humans and seals. While it does not cause pandemics like influenza A, it can still lead to seasonal flu epidemics. Unlike influenza A, influenza B does not undergo the same level of genetic variation but can still cause significant illness, particularly in children and the elderly.
Influenza C virus causes mild respiratory illness and is less common in humans. It is typically not responsible for widespread outbreaks, and its symptoms are often less severe than those caused by influenza A or B.
Influenza D virus primarily affects cattle and does not pose a threat to human health. It was first identified in 2011 and is not known to cause human disease.
In summary, human influenza can be caused by the influenza A, B, or C viruses. Influenza A is the most dangerous and responsible for pandemics, while B causes seasonal outbreaks. Influenza C leads to milder illness and is less common, while influenza D is not a human pathogen.