Learn Russian: Грипп — это болезнь.

In Russian, the present tense verb for 'to be' is skipped. When you connect two nouns to say one thing is another thing (like 'Flu is an illness'), you typically use a long dash followed by это to bridge them together.

Since болезнь ends in a soft sign (-ь), how do I know if it is masculine or feminine?

Болезнь (illness) is feminine. Words ending in a soft sign can be either masculine or feminine, so you usually have to memorize their gender. However, a helpful trick is that Russian nouns ending in -знь, like жизнь (life) and болезнь, are always feminine.

Why does грипп end with a double п?

Грипп is a foreign word borrowed from French. Borrowed words in Russian frequently keep their original double consonants. You do not need to pronounce it as two separate sounds; just say a normal п.