Viral diseases — AQA GCSE Biology

Because measles is highly contagious and can cause serious or fatal complications, most young children are vaccinated against it. The vaccine used in the UK is the MMR vaccine, which protects against measles, mumps and rubella. It contains a weakened or inactivated form of the virus that stimulates the immune system to produce antibodies and memory cells without causing the disease. If the vaccinated child later encounters the real measles virus, the immune system responds rapidly and prevents illness. High vaccination rates also create herd immunity, protecting those who cannot be vaccinated, such as very young babies or people with certain medical conditions. This is why vaccination is offered routinely in early childhood and why maintaining high uptake is important for public health.

Measles is a viral disease transmitted when an infected person sneezes or coughs, releasing tiny liquid droplets into the air. These droplets contain virus particles and are inhaled by nearby people, allowing the virus to enter the respiratory tract. This is an airborne droplet route, not direct skin contact or a food-borne route. A useful classroom model is a spray bottle: a fine mist released in one place can be breathed in some distance away, just as droplets travel through the air. Because transmission happens before or during early symptoms, isolation and good respiratory hygiene reduce spread. The measles virus is not a bacterium, so antibiotics do not treat it; vaccination is the main control measure.

When a person first becomes infected with HIV, the virus multiplies in the body and the immune system responds. During this early stage many people develop symptoms similar to influenza, such as fever, sore throat, tiredness, headache and swollen glands. These symptoms are not caused by influenza virus; they are the body's early response to HIV. The flu-like illness may be mild and short-lived, so it can easily be mistaken for a common cold or flu. After this initial phase, the person may feel well for years while the virus continues to damage immune cells. A blood test, not the symptoms alone, is needed to confirm HIV infection.

After the initial flu-like illness, HIV continues to reproduce inside the body. If the infection is not controlled by antiretroviral drugs, the virus attacks immune cells, especially helper T cells. These cells coordinate the immune response, so their destruction weakens the body's ability to fight other pathogens. Antiretroviral drugs do not cure HIV, but when taken correctly they suppress viral replication and allow the immune system to remain functional. This is why treatment must be continued: stopping treatment allows the virus to multiply and attack immune cells again. A useful analogy is a security system losing its control room operators, so other defences cannot be coordinated effectively.

AIDS is the late stage of HIV infection. It develops when HIV has destroyed so many immune cells that the immune system can no longer coordinate effective responses. At this point, infections that would normally be harmless, called opportunistic infections, can cause serious illness. Some cancers also become more likely because the immune system normally destroys abnormal cells. AIDS is not a single disease with one set of symptoms; it is a syndrome defined by severe immune damage and the resulting infections or cancers. With effective antiretroviral treatment, HIV need not progress to AIDS, so late stage HIV is not inevitable.

HIV is a virus that infects humans and, if untreated, can lead to AIDS by destroying lymphocytes so the immune system fails. It is not spread by everyday contact such as hugging, sharing food or mosquito bites. Transmission requires direct transfer of infected body fluids into the bloodstream or mucous membranes. This happens during unprotected sexual contact, when drug users share needles and traces of infected blood pass from one person to another, or through blood transfusion and from mother to child. Understanding these routes explains why needle-exchange schemes, condom use and screening donated blood reduce spread. The virus itself does not kill the host directly; it leaves the body unable to fight other pathogens.

TMV is transmitted mechanically, not by vectors. Tobacco mosaic virus is a plant pathogen affecting many species, including tomatoes. It is described as widespread because it occurs in many regions and survives in plant debris, soil, and sap. It damages commercial glasshouse crops and garden plants. The virus spreads when sap containing virus particles enters a plant through tiny wounds, for example during handling or pruning. Once inside, it hijacks the plant's cells to make more virus particles. Because it is a virus, antibiotics are useless; control relies on removing infected plants, disinfecting tools, and using resistant varieties.

When tobacco mosaic virus infects a plant, leaf cells become damaged and discoloured, producing a patchwork of light and dark areas described as a mosaic pattern. The pale patches contain less chlorophyll, the green pigment that absorbs light for photosynthesis. Because photosynthesis is reduced, less glucose is made, so the plant has less energy and material for growth. The plant may be stunted, produce fewer or smaller leaves and yield less fruit. This links the visible symptom directly to a cellular cause: damaged cells mean less chlorophyll, less photosynthesis, less growth. A useful method is to compare an infected leaf with a healthy leaf and note the pale patches, then explain the consequence for glucose production and growth.