Introduction:
The narrator was haunted by the idea of a toothbrush while travelling due to several reasons.

Unavailability of a Toothbrush:
The first reason for the narrator being haunted by the idea of a toothbrush while travelling is the unavailability of a toothbrush. The narrator was travelling in a remote area where basic amenities were not easily available. Hence, he did not have access to a toothbrush, which made him feel uncomfortable.

Importance of Oral Hygiene:
The second reason for the narrator being haunted by the idea of a toothbrush while travelling is the importance of oral hygiene. Oral hygiene is essential for maintaining good health, and not being able to brush his teeth made the narrator feel unclean and unhealthy. The narrator was worried about the accumulation of bacteria and plaque in his mouth, which could lead to dental problems.

Past Experiences:
The third reason for the narrator being haunted by the idea of a toothbrush while travelling is past experiences. The narrator might have faced dental problems due to the lack of proper oral hygiene in the past, and hence, he was more conscious about it.

Conclusion:
In conclusion, the narrator was haunted by the idea of a toothbrush while travelling due to the unavailability of a toothbrush, the importance of oral hygiene, and past experiences. It is essential to maintain proper oral hygiene while travelling to avoid dental problems.

The “horrible idea” that occurred to Jerome a little later was wether he had packed his toothbrush or not. Whenever he travelled he could never remember if he had packed his toothbrush or not. This thought haunted him and made his life a misery.