flu

As a letter s

is the letter s with a.

As a noun flu is

influenza.

As nouns the difference between flu and null

is that flu is influenza while null is zero, nil; the cardinal number before einn.

As nouns the difference between flu and sick

is that flu is influenza while sick is sick people in general as a group.

As an adjective sick is

in poor health.

As a verb sick is

to vomit or sick can be (rare).

As nouns the difference between clue and flu

is that clue is a strand of yarn etc as used to guide one through a labyrinth; something which points the way, a guide while flu is influenza.

As a verb clue

is to provide with a clue.

As a noun flu

is influenza.

As an initialism rsv is

revised standard version (a version of the bible translated into english).

As a noun flu

is influenza.

As nouns the difference between pandemic and flu

is that pandemic is a pandemic disease; a disease that hits a wide geographical area and affects a large proportion of the population while flu is influenza.

As an adjective pandemic

is widespread; general.

As nouns the difference between flu and sickness

is that flu is influenza while sickness is the quality or state of being sick or diseased; illness; disease or malady.

As nouns the difference between adenovirus and flu

is that adenovirus is any virus of the family , many of which are responsible for respiratory infections in humans while flu is influenza.

As nouns the difference between hangover and flu

is that hangover is illness caused by a previous bout of alcohol drinking while flu is influenza.