Frederick Augustus, Duke of Nassau
Friedrich August, Duke of Nassau, Prince of Nassau- Usingen( 23 April 1738 in Usingen – 24 March 1816 in Wiesbaden) was the last Prince of Nassau- Usingen and, jointly with his cousin, Friedrich Wilhelm of Nassau- Weilburg, first Duke of Nassau.
He died without surviving male issue and was succeeded by his cousin's son, Wilhelm.
Princess Marie of Waldeck and Pyrmont
Princess Marie of Waldeck and Pyrmont (Georgine Henriette Marie; 23 May 1857 – 30 April 1882) was the third daughter of George Victor, Prince of Waldeck and Pyrmont and his wife, Princess Helena of Nassau, younger half-sister of Adolphe, Grand Duke of Luxembourg.
Duke Friedrich August of Oldenburg
Duke Friedrich August of Oldenburg( 11 January 1936- 9 July 2017) was a member of the House of Holstein- Gottorp.
He is a son of Nikolaus, Hereditary Grand Duke of Oldenburg and Princess Helena of Waldeck and Pyrmont.
George Victor, Prince of Waldeck and Pyrmont
George Victor (14 January 1831 – 12 May 1893) was the 3rd sovereign Prince of the German state of Waldeck and Pyrmont.
He was born in Arolsen the son of George II, Prince of Waldeck and Pyrmont and his wife Princess Emma of Anhalt-Bernburg-Schaumburg-Hoym.
He succeeded as prince originally under the guardianship of his mother on 15 May 1845 following the death of his father.
He died of pneumonia in Marienbad, Bohemia and was succeeded by his eldest son Friedrich.
Henry I, Count of Nassau-Siegen
Henry I of Nassau- Siegen,( – between 13 July and 14 August 1343) was Count of Nassau- Siegen, a part of the County of Nassau, and ancestor of the House of Nassau- Siegen.
He comes from the Ottonian branch of the House of Nassau.
Elisabeth of Nassau-Dillenburg, Countess of Wied
Elisabeth of Nassau-Dillenburg (1 December 1488 – 3 June 1559 in Dillenburg) was a member of the House of Nassau.
She was a daughter of John V of Nassau-Dillenburg and his wife Elisabeth of Hesse-Marburg.
Her brothers were Henry III of Nassau-Breda, John of Nassau-Vianden-Dietz, Ernest of Nassau and William the Rich.
Her sister was Maria of Nassau.
Otto I, Count of Nassau
Otto I of Nassau,( † between 3 May 1289 and 19 March 1290) was Count of Nassau and is the ancestor of the Ottonian branch of the House of Nassau.
Walram II, Count of Nassau
Walram II of Nassau,( – 24 January 1276), was Count of Nassau and is the ancestor of the Walramian branch of the House of Nassau.
Princess Helena of Nassau
Princess Helena of Nassau (18 August 183127 October 1888) was a daughter of William, Duke of Nassau, and consort of George Victor, Prince of Waldeck and Pyrmont.
Princess Helena of Waldeck and Pyrmont
Princess Helena of Waldeck and Pyrmont( Helene Friederike Auguste; later Duchess of Albany; 17 February 1861 – 1 September 1922), who became a member of the British royal family by marriage, was the fifth daughter and child of George Victor, Prince of Waldeck and Pyrmont and his first wife, Princess Helena of Nassau.

Question: Why did Princess Helena Of Nassau's husband die?
Answer: pneumonia