Communicable Diseases » Bacterial, Fungal and Protist Diseases

Bacteria, fungi and protists that cause disease

In the last lesson you met three diseases caused by viruses. Here are four more diseases: two are caused by bacteria, one by a fungus and one by a protist. For each one you need to know how it spreads, the symptoms it causes and how its spread is controlled.

Key terms:

• Salmonella food poisoning: a bacterial disease spread by bacteria eaten in food.
• Gonorrhoea: a sexually transmitted disease (STD) caused by a bacterium.
• Rose black spot: a fungal disease of plants that causes purple or black spots on leaves.
• Malaria: a disease caused by a protist, spread by mosquitos.
• Fungicide: a chemical used to kill fungi.

Salmonella food poisoning

Salmonella food poisoning is caused by bacteria. It is spread when people eat the bacteria in food, or eat food that was prepared in unhygienic conditions. For example, food might be prepared on a dirty surface or with unwashed hands.

The symptoms are fever, abdominal cramps, vomiting and diarrhoea. They are caused by the bacteria themselves and by the toxins that the bacteria secrete.

Control: in the UK, poultry (farm birds such as chickens) are vaccinated against Salmonella. This helps to control the spread of the disease, because fewer birds carry the bacteria into our food.

Gonorrhoea

Gonorrhoea is a sexually transmitted disease (STD). It is caused by a bacterium and is spread by sexual contact.

The symptoms are a thick yellow or green discharge from the vagina or penis, and pain on urinating.

Treatment: gonorrhoea used to be easily treated with the antibiotic penicillin. Many resistant strains have now appeared, so penicillin no longer works on them. (You will see how resistance happens in the lesson on extinction and resistant bacteria.)

Control: the spread can be controlled in two ways:

• treating infected people with antibiotics
• using a barrier method of contraception, such as a condom, which stops the bacteria passing from one person to another

Rose black spot

Rose black spot is a fungal disease of plants. Purple or black spots develop on the leaves. The leaves often turn yellow and drop early.

This affects the growth of the plant, because photosynthesis is reduced. Fewer healthy leaves means less light is absorbed and less glucose is made.

The fungus is spread in the environment by water or wind. For example, rain splashing from leaf to leaf can carry it to a new plant.

Treatment: rose black spot can be treated by:

• using fungicides
• removing and destroying the affected leaves

Malaria

The pathogens that cause malaria are protists. The malarial protist has a life cycle that includes the mosquito. The mosquito is the vector: it carries the pathogen from one person to the next.

Malaria causes recurrent episodes of fever. That means the fever keeps coming back. Malaria can be fatal.

Control: the spread of malaria is controlled by targeting the vector:

• preventing the mosquitos from breeding, so there are fewer of them
• using mosquito nets to avoid being bitten

Comparing the four diseases

Exam-style question

(a) Name the type of pathogen that causes malaria. [1 mark]

(b) Describe how the spread of malaria is controlled. [2 marks]

(c) A gardener sees purple spots on a rose leaf. The leaf turns yellow and falls off. Explain why the rose grows less well, and suggest how the gardener could treat the disease. [3 marks]