This is the 10157th most frequent Swedish word.
influensavirus
Influenza virus.
Here, 'influensaviruset' is used as a noun in definite form to refer to a specific virus.
Influensaviruset orsakar ofta hög feber och hosta.
The influenza virus often causes high fever and coughing.
In this sentence, 'influensavirus' is used as a noun in the indefinite form, referring generically to influenza viruses.
Vaccinationer kan skydda oss mot vissa influensavirus.
Vaccinations can protect us against certain influenza viruses.
Here, 'influensavirus' is used as a singular noun to describe the general cause of the flu during the season.