Relapsing fever is caused by which genus of bacteria?

• A. Borrelia
• B. Treponema
• C. Leptospira
• D. Spirillum

Answer: (A)

Relapsing fever is caused by spirochetes of the genus Borrelia. These bacteria circulate in the blood and show antigenic variation, meaning their surface proteins change over time and allow a portion of the bacterial population to evade the host immune response. This immune evasion leads to successive fever spikes as each new wave of bacteria enters and the immune system responds. Borrelia recurrentis (louse-borne) and various tick-borne Borrelia species cause the disease. The other genera are associated with different infections: Treponema with treponemal diseases like syphilis, Leptospira with leptospirosis, and Spirillum is not typically linked to relapsing fever.