**Question:** At present following types of influenza viruses are circulating in the world, except -

A. Influenza A(H7N9)
B. Influenza B
C. Influenza C
D. Influenza D

**Correct Answer: D. Influenza D**

**Core Concept:**
Influenza viruses are a group of RNA viruses that cause infectious respiratory illness known as influenza. The pandemic caused by the novel coronavirus (SARS-CoV-2) has attracted significant attention, but other influenza viruses, such as A, B, and C, still circulate globally. Influenza D, also known as bovine influenza or calici virus, primarily affects cattle and is not typically associated with human infections.

**Why the Correct Answer is Right:**
Influenza D is specific to cattle and does not cause human infections. Unlike influenza A, B, and C, which can infect humans and lead to influenza-like illnesses, D virus remains confined to the bovine population. The reason for this is the specific host range and tropism of the virus, which targets and replicates within the respiratory, gastrointestinal, and sometimes neurological systems of cattle.

**Why Each Wrong Option is Incorrect:**

A. Influenza A(H7N9) is a subtype of influenza A virus, which can infect humans and is responsible for severe respiratory illnesses. Influenza A(H7N9) emerged in 2013 in China and was a cause for concern due to its potential pandemic threat.

B. Influenza B is another subtype of influenza A virus, which causes human infections and influenza-like illnesses. Influenza B virus is known for causing pandemic outbreaks, such as the Hong Kong flu in 1968.

C. Influenza C is another subtype of influenza virus that can cause mild respiratory illness in humans. Influenza C virus is less severe compared to influenza A and B viruses but still has clinical implications.

**Clinical Pearl:**
Influenza D virus is an important clinical distinction because it helps differentiate between human influenza infections caused by A, B, and C viruses and bovine infections. This knowledge is crucial for clinicians treating patients with respiratory illnesses, as it guides appropriate diagnostics, treatment, and public health measures.

By understanding the specific host range and tropism of different influenza virus subtypes, physicians can correctly diagnose and manage patients with influenza-like illnesses and implement appropriate public health measures to prevent the spread of pandemic-prone subtypes.