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Unlock This CourseViruses are tiny infectious agents that can only reproduce inside living cells. They're much smaller than bacteria and cause some of the world's most serious diseases. Unlike bacteria, viruses aren't technically alive - they're more like biological machines that hijack our cells to make copies of themselves.
Key Definitions:
Viruses can't survive or reproduce on their own. They must invade living cells and use the cell's machinery to make copies of themselves. This process often damages or destroys the host cell, causing disease symptoms. The new viruses then spread to infect other cells.
Viral diseases affect millions of people worldwide and range from mild infections like the common cold to life-threatening conditions. Understanding these diseases helps us prevent their spread and develop better treatments.
Human Immunodeficiency Virus (HIV) attacks the immune system, specifically targeting T-helper cells. If left untreated, it leads to Acquired Immunodeficiency Syndrome (AIDS), where the body can't fight off infections.
Spreads through blood, sexual contact and from mother to child during pregnancy or breastfeeding. Cannot spread through casual contact, saliva, or sharing food.
Early stages may have flu-like symptoms. Later stages include severe weight loss, recurring infections and certain cancers as the immune system fails.
Antiretroviral drugs can control the virus and prevent progression to AIDS. With proper treatment, people with HIV can live normal lifespans.
Countries like Botswana have dramatically reduced HIV transmission rates through comprehensive education programmes, free testing and widespread access to antiretroviral drugs. Mother-to-child transmission has dropped from over 40% to less than 2% in many regions with proper medical intervention.
Influenza viruses cause seasonal epidemics and occasional pandemics. The virus constantly changes its surface proteins, making it difficult for our immune system to recognise and fight new strains.
Occurs annually during winter months. Symptoms include fever, body aches, cough and fatigue. Most people recover within a week, but it can be serious for elderly people and those with chronic conditions.
Occurs when a new flu strain emerges that humans have no immunity against. The 1918 Spanish flu killed 50-100 million people worldwide. More recent pandemics include H1N1 in 2009 and the ongoing threat of H5N1 bird flu.
Once a common childhood disease, measles is now preventable through vaccination. However, it remains a leading cause of death in young children in areas with poor vaccination coverage.
Characteristics of Measles:
Viruses don't just affect animals - they also cause serious diseases in plants, leading to significant crop losses and economic damage worldwide.
One of the most studied plant viruses, TMV affects tobacco, tomatoes and other plants in the nightshade family. It was the first virus ever discovered in 1892.
Creates a distinctive mosaic pattern of light and dark green patches on leaves. Infected plants may have stunted growth and reduced crop yields.
Spreads through direct contact, contaminated tools, or insects. The virus can survive in soil and plant debris for extended periods.
Causes millions of pounds in crop losses annually. Particularly devastating for tobacco farmers and greenhouse tomato growers.
In sub-Saharan Africa, cassava mosaic virus threatens food security for over 300 million people who depend on cassava as a staple crop. Scientists have developed virus-resistant cassava varieties through genetic modification and selective breeding, helping farmers maintain yields despite viral infections.
Understanding transmission methods is crucial for preventing viral diseases. Different viruses use various strategies to move between hosts.
Viruses like influenza and measles spread through respiratory droplets when infected people cough, sneeze, or talk. These droplets can travel several metres and infect others who breathe them in.
Some viruses spread through direct physical contact with infected individuals or contaminated surfaces. Examples include cold sores (herpes simplex) and some stomach bugs (norovirus).
Certain viruses use insects or other animals as carriers. Mosquitoes transmit dengue fever and Zika virus, while ticks can spread tick-borne encephalitis.
Preventing viral diseases is often more effective than treating them after infection occurs. Unlike bacterial infections, viral diseases don't respond to antibiotics.
The most effective ways to prevent viral diseases include vaccination, good hygiene practices and avoiding exposure to infected individuals or vectors.
Key Prevention Methods:
While antibiotics don't work against viruses, several treatment approaches can help manage viral infections and support recovery.
Specific medications that target viral replication. Examples include antiretroviral drugs for HIV, oseltamivir for influenza and aciclovir for herpes infections. These drugs work best when started early in the infection.
Managing symptoms while the immune system fights the virus. This includes rest, fluids, pain relief and treating secondary bacterial infections that may occur when the immune system is weakened.
Antibiotics target specific structures in bacterial cells, such as cell walls or protein-making machinery. Since viruses don't have these structures and aren't technically alive, antibiotics have no effect on them. Using antibiotics unnecessarily can lead to antibiotic resistance in bacteria, making future bacterial infections harder to treat.