Infectious diseases are caused by

1. Analyze the question The question asks what causes infectious diseases. The options are bacteria, protists, Amoeba and protozoa. Several of them are specific kinds of pathogen, so we need to find the one that fits the question best.

2. Explain the concept An infectious disease is caused by a pathogen that can spread from one host to another. Pathogens include:

• Bacteria, e.g. cholera, typhoid, tuberculosis
• Viruses, e.g. HIV, measles
• Fungi, e.g. ringworm
• Protozoa, e.g. malaria (Plasmodium) and sleeping sickness (Trypanosoma)

3. Work through the options

• A. Bacteria: These are well-known causes of infectious diseases such as cholera, typhoid and tuberculosis. This is the standard textbook answer.
• B. Protists: This is a broad kingdom, and only some members are pathogenic. It is too general to be the best answer.
• C. Amoeba: This is a single genus. Only certain species, such as Entamoeba histolytica, cause disease (amoebic dysentery). It is too specific.
• D. Protozoa: These do cause diseases like malaria, but they are only one group of pathogens, just as bacteria are.

Strictly speaking, B, C and D can also cause infectious diseases, so the question is not perfectly worded. However, JAMB's expected answer is bacteria, since they are the classic and most widely recognised cause of infectious disease.

4. Answer A. bacteria

5. Exam tip When several options seem partly correct, choose the most general, standard textbook answer. Also, remember the pathogen groups with examples:

• Bacteria: cholera, typhoid, TB
• Protozoa: malaria, sleeping sickness, amoebic dysentery
• Viruses: HIV, polio, measles
• Fungi: ringworm, athlete's foot