Foudroie comes from the verb foudroyer, meaning 'to strike down' or 'to smite' with lightning. It's a dramatic and less common verb that evokes a sudden, powerful, and often overwhelming impact.
In the song, it's used to describe how love (la maladie d'amour) can suddenly and intensely strike someone, like "Elle foudroie dans la rue cet inconnu qui passe" (It strikes down in the street this stranger who passes). This vivid imagery makes the word particularly memorable and impactful.
In his classic song, French singer Michel Sardou describes love not as a feeling, but as "la maladie d'amour"—the sickness of love. But don't worry, it's a sickness he suggests everyone catches! According to the lyrics, this "illness" runs through the hearts of everyone from age seven to seventy-seven, uniting young and old people ("blonde hair, grey hair") like a powerful river.
This "sickness" has two sides. On one hand, it's a wonderful force that makes people sing and makes the world feel bigger. On the other hand, it can cause a lifetime of suffering, making people cry and scream in the shadows. But the song's most powerful idea is that the most painful thing of all isn't having this sickness, but being cured of it. It suggests that a life with the joys and pains of love is far better than a life without it.