What is the 'flu pathogen'?

GOOGLE: There are three types of influenza viruses: A, B, and C. Major epidemics are caused by A and B, and it is usually these two varieties that cause the flu. They have a diameter of about 100 nanometers, making them a medium sized virus.

MORE GOOGLE: Influenza is caused by infection of the respiratory tract with influenza viruses, RNA viruses of the Orthomyxovirus genus. Influenza viruses are classified into 4 types: A, B, C, and D. Only virus types A and B commonly cause illness in humans.

ONE MORE GOOGLE: In the past, it was thought that infection with influenza A was more severe than infection with influenza B. However, a 2015 study in adults with influenza A and influenza B found they both resulted in similar rates of illness and death.

I hope this helps! I honestly don't know what the flu pathogen is either but I guess Google might have helped you a bit..? Sorry I couldn't answer properly. Goo luck!

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.