Nosferatu

Thomas Hutter lives in the (fictitious) German city of Wisborg. His employer, Knock, sends Hutter to Transylvania to visit a new client named Count Orlok. Hutter entrusts his loving wife Ellen to his good friend Harding and Harding's sister Annie, before embarking on his long journey. Nearing his destination in the Carpathian mountains, Hutter stops at an inn for dinner. The locals become frightened by the mere mention of Orlok's name and discourage him from traveling to his castle at night, warning of a werewolf on the prowl. The next morning, Hutter takes a coach to a high mountain pass, but the coachmen decline to take him any further than the bridge as nightfall is approaching. A black-swathed coach appears after Hutter crosses the bridge and the coachman gestures for him to climb aboard. Hutter is welcomed at a castle by Count Orlok. When Hutter is eating dinner and accidentally cuts his thumb, Orlok tries to suck the blood out, but his repulsed guest pulls his hand away.
Hutter wakes up to a deserted castle the morning after and notices fresh punctures on his neck, which he attributes to mosquitoes in his letter to his wife which he hands over to a courier on horseback to be delivered to his devoted wife. That night, Orlok signs the documents to purchase the house across from Hutter's own home and notices a photo of Hutter's wife remarking that she has a lovely neck. Reading a book about vampires that he took from the local inn, Hutter starts to suspect that Orlok is Nosferatu, the "Bird of Death." He cowers in his room as midnight approaches, but there is no way to bar the door. The door opens by itself and Orlok enters, his true nature finally revealed, and Hutter hides under the bedcovers and falls unconscious. At the same time this is happening, the wife awakens from her sleep and in a trance walks towards the balcony and onto the railing. Alarmed, Harding shouts Ellen's name and she faints while he asks for a doctor. After the doctor arrives, she shouts Hutter's name remaining in the trance and apparently able to see Orlok in his castle threatening her unconscious husband. The doctor believes this trance-like state is due to "blood congestion". The next day, Hutter explores the castle. In its crypt, he finds the coffin in which Orlok is resting dormant. Hutter becomes horrified and dashes back to his room. Hours later from the window, he sees Orlok piling up coffins on a coach and climbing into the last one before the coach departs. Hutter escapes the castle through the window, but is knocked unconscious by the fall and awakes in a hospital.




An iconic scene of the shadow of Count Orlok climbing up a staircase
When he is sufficiently recovered, he hurries home. Meanwhile, the coffins are shipped down river on a raft. They are transferred to a schooner, but not before one is opened by the crew, revealing a multitude of rats. The sailors on the ship get sick one by one; soon all but the captain and first mate are dead. Suspecting the truth, the first mate goes below to destroy the coffins. However, Orlok awakens and the horrified sailor jumps into the sea. Unaware of his danger, the captain becomes Orlok's latest victim when he ties himself to the wheel. When the ship arrives in Wisborg, Orlok leaves unobserved, carrying one of his coffins, and moves into the house he purchased. The next morning, when the ship is inspected, the captain is found dead. After examining the logbook, the doctors assume they are dealing with the plague. The town is stricken with panic, and people are warned to stay inside.
There are many deaths in the town, which are blamed on the plague. Knock, who had been committed to a psychiatric ward, escapes after murdering the warden. The townspeople give chase, but he eludes them by climbing a roof, then using a scarecrow. Meanwhile, Orlok stares from his window at the sleeping Ellen. Against her husband's wishes, Ellen had read the book he found. The book claims that the way to defeat a vampire is for a woman who is pure in heart to distract the vampire with her beauty all through the night. She opens her window to invite him in, but faints. (In a deleted scene, the actress who played the hero's sister, Ruth Landshoff was featured in this scene, where was she fleeing from the vampire, running along a beach. That scene is not in any version or restoration of the film, nor in the original script). When Hutter revives her, she sends him to fetch Professor Bulwer. After he leaves, Orlok comes in. He becomes so engrossed drinking her blood that he forgets about the coming day. When a rooster crows, Orlok vanishes in a puff of smoke as he tries to flee. Ellen lives just long enough to be embraced by her grief-stricken husband. The last scene shows Count Orlok's ruined castle in the Carpathian Mountains, symbolizing the end of Count Orlok's reign of terror.

Question: What do the doctors assume has been the cause of the sailors' death after looking at the logbook?
Answer: the plague