This is the 2676th most frequent German word.
erkranken
To fall ill.
Here, 'erkranken' is used in the past tense to indicate that the subject became ill with a certain condition, specifying 'Grippe' (flu).
Er ist an einer Grippe erkrankt.
He fell ill with the flu.
In this example, 'erkranken' is employed to describe the possibility of becoming ill, with 'an vielen verschiedenen Krankheiten' indicating various illnesses.
Menschen können an vielen verschiedenen Krankheiten erkranken.
People can fall ill with many different diseases.
This usage of 'erkranken' conveys the idea that the subject became significantly unwell after an event, emphasizing the severity with 'schwer' (seriously).