Viral diseases

Infection and response · Communicable diseases · note 2 of 9

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Viral diseasesSpec 4.3.1.2

In short

Measles, HIV and tobacco mosaic virus are viral diseases. Measles causes a fever and a red skin rash, is spread by inhaling droplets from sneezes and coughs, and is prevented by vaccination. HIV attacks immune cells unless controlled with antiretroviral drugs, leading to AIDS. TMV gives a mosaic pattern on leaves, reducing photosynthesis and growth.

Measles

Measles is a viral disease. The symptoms are a fever and a red skin rash. It is spread by inhalation of droplets from sneezes and coughs.

Measles is a serious illness that can be fatal if complications arise. For this reason most young children are vaccinated against measles.

HIV

HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs, the virus attacks the body's immune cells.

Late stage HIV infection, or AIDS, happens when the immune system becomes so badly damaged that it can no longer deal with other infections or cancers.

HIV is spread by sexual contact or by the exchange of body fluids such as blood, which happens when drug users share needles.

Common mistake:

HIV is the virus. AIDS is the late stage of the infection. Do not say that HIV is 'the same as' AIDS, and do not say that antiretroviral drugs cure it. They control the virus.

Tobacco mosaic virus (TMV)

TMV is a widespread plant pathogen. It affects many species of plants, including tomatoes.

It gives a distinctive mosaic pattern of discolouration on the leaves. This affects the growth of the plant because there is less photosynthesis.

A healthy leaf of even green colour next to a leaf infected with tobacco mosaic virus, showing a mosaic of light and dark green patches. — TMV gives leaves a **mosaic** of light and dark patches, so there is less photosynthesis.

Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026

Frequently asked questions

How do pathogens spread?

Pathogens spread by direct contact, by water or by air. HIV and gonorrhoea spread by sexual contact, and HIV also by the exchange of body fluids such as blood. Measles spreads in droplets from sneezes and coughs, and rose black spot by water or wind. Salmonella spreads in food, and malaria is carried by mosquitos.

How do white blood cells fight infection?

White blood cells fight infection in three ways. Some engulf and digest pathogens, which is called phagocytosis. Others produce antibodies, which target a particular pathogen and help to destroy it, and antitoxins, which neutralise the toxins released by pathogens. This is the immune system, which acts when a pathogen gets past the non-specific defences.

How do vaccines work?

Vaccines work by introducing small quantities of dead or inactive pathogen into the body, which stimulates white blood cells to produce antibodies. If the same pathogen enters the body later, the white blood cells respond quickly to make the correct antibodies, so the pathogen is destroyed before it causes illness. A vaccine does not contain antibodies itself.

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