Viral diseases
HIV infection and AIDS
HIV is a virus that damages cells involved in the body’s defence against disease. Over time, this damage can weaken the immune system, making it harder for the body to fight infections.
If the immune system becomes severely damaged, HIV infection may progress to acquired immunodeficiency syndrome (AIDS). A person with AIDS is especially vulnerable to other infections because their immune defences are severely weakened.
HIV and AIDS are not the same thing. HIV is the pathogen; AIDS is a possible later condition caused by the effects of HIV on the immune system. Not everyone with HIV develops AIDS.
Disease transmission
Communicable diseases can pass between hosts. In animals and plants, these diseases may be caused by pathogens: viruses, bacteria, protists or fungi. A host is an organism that a pathogen can infect.
Transmission occurs when a pathogen passes from an infected host or a contaminated source to another host. If the pathogen enters and establishes itself in the new host, it can cause an infection. The route by which it travels depends on the pathogen and the host.
Routes of disease transmission
A pathogen can reach a new host by several routes:
- Direct contact: the pathogen transfers when hosts touch.
- Water: water carrying the pathogen reaches a new host.
- Air: droplets or particles carrying the pathogen reach animal or plant tissues.
Breaking a transmission route
A pathogen has fewer opportunities to cause infection if its route to a susceptible host is interrupted. For example, isolating an infected host reduces its contact with other hosts. Removing infected plants can likewise limit contact between a pathogen and plants that could be infected.
Measles
Measles is a viral disease that can cause fever and a red skin rash. It can be serious, and complications can sometimes be fatal.
An infected person releases droplets when they cough or sneeze. Another person can inhale these droplets, allowing the virus to enter their body and potentially cause infection.
Young children are commonly vaccinated against measles to protect them from the disease.
HIV transmission
HIV can pass from one person to another when a body fluid containing the virus enters the other person’s body.
During sexual contact, fluids such as semen, vaginal fluid or blood containing HIV can enter a partner’s body through the lining of the vagina, rectum or other openings, or through breaks in the skin.
HIV can also be transmitted when people share needles. A needle used by a person with HIV may contain traces of their blood. If another person uses it, the infected blood can enter their bloodstream.
In both routes, transmission involves the transfer of a body fluid containing HIV from one person to another.
Antiretroviral treatment of HIV
HIV infection can initially cause a flu-like illness. These symptoms may subside, but the virus remains in the body. HIV can continue to replicate and damage cells of the immune system, which normally helps the body deal with infections.
Antiretroviral drugs suppress HIV replication. This reduces the amount of virus in the body and helps protect immune cells from damage. With effective, ongoing treatment, HIV infection can be controlled and progression to AIDS can be prevented.
Without successful control of HIV, damage to the immune system can become so severe that the body can no longer deal effectively with other infections or certain cancers. This advanced stage is called AIDS.
Tobacco mosaic virus
Tobacco mosaic virus (TMV) is a widespread plant pathogen: a virus that causes disease in plants. It infects many plant species, including tomato plants.
Infected leaves develop a distinctive mosaic pattern of discolouration. The affected leaves carry out less photosynthesis, so the plant makes less glucose for growth. As a result, the plant grows less.