Cambridge A level biology textbook-pages-2

Chapter 10: Infectious Diseases

Learning Intentions

  • Understand that infectious diseases are caused by pathogens transmitted from person to person.

  • Identify the pathogens causing cholera, malaria, tuberculosis (TB), and HIV/AIDS and explain their modes of transmission.

  • Recognize that human pathogens can include viruses, bacteria, and protoctists.

  • Analyze the biological, social, and economic factors influencing control measures for cholera, malaria, TB, and HIV.

  • Explain the mechanism of action of the antibiotic penicillin on bacteria.

  • Distinguish reasons why antibiotics are ineffective against viruses.

  • Discuss the consequences of pathogen resistance to antibiotics.

  • Outline measures to mitigate the impact of antibiotic resistance.

Understanding Infectious Diseases

Definition

  • Infectious Diseases: Diseases caused by pathogens, often called communicable diseases, transmitted from infected individuals to uninfected ones.

  • Transmission can be direct or indirect, and some pathogens can survive outside the human body.

Symptoms and Duration

  • Infectious diseases can exhibit a range of severities and durations; some like the common cold are short-lived, while others like HIV/AIDS require lifelong management.

Disease Carriers

  • Carriers: Individuals who harbor a pathogen and can transmit it without showing symptoms.

Transmission Cycle

  • Understanding the cycle of transmission is crucial for breaking the spread of diseases.

Key Infectious Diseases

1. Cholera

  • Pathogen: Vibrio cholerae (bacterium)

  • Transmission: Waterborne, especially in areas lacking sanitation. Symptomless carriers can spread the bacteria via contaminated water or food.

  • Control Measures: Vaccination, rehydration therapy, sanitation improvements, and access to clean water.

2. Malaria

  • Pathogens: Plasmodium species (protoctist)

  • Transmission: Female Anopheles mosquitoes; can also spread through blood transfusions, unsterile needles, and mother-to-child (placenta).

  • Symptoms: Fever, anemia, nausea, headaches; can lead to serious complications if untreated.

  • Control Measures: Insecticide-treated nets, environmental management, and use of antimalarial drugs such as quinine and artemisinin-based therapies.

3. HIV/AIDS

  • Pathogen: Human Immunodeficiency Virus (HIV)

  • Transmission: Through sexual contact, sharing needles, blood transfusions, and from mother to child.

  • Symptoms: Initially flu-like, advancing to a weakened immune system leading to opportunistic infections.

  • Control Measures: Antiretroviral therapy, education on safe practices, condoms, and contact tracing.

4. Tuberculosis (TB)

  • Pathogens: Mycobacterium tuberculosis and Mycobacterium bovis

  • Transmission: Airborne droplets from coughing, especially in overcrowded conditions.

  • Symptoms: Persistent cough, chest pain, fever, weight loss.

  • Control Measures: Long-term antibiotic treatment, isolation of active cases, and BCG vaccination.

Antibiotics and Resistance

Mechanism of Action

  • Antibiotics: Compounds that inhibit bacterial growth or kill bacteria without harming human cells.

  • Example: Penicillin inhibits cell wall synthesis in bacteria, but does not affect viruses.

Antibiotic Resistance

  • Resistance can develop through mutations and the acquisition of resistance genes, often facilitated by inappropriate use of antibiotics.

  • Consequences: Increased difficulty in treating infections, leading to more severe health outcomes and higher mortality rates.

Reducing Antibiotic Resistance

  • Strategies include:

    • Limiting antibiotic prescriptions to necessary cases.

    • Encouraging completion of prescribed courses.

    • Promoting narrow-spectrum antibiotics over broad-spectrum ones.

    • Restricting antibiotics in agriculture.

Conclusion

Understanding the mechanisms, transmission modes, and control measures for infectious diseases, along with the implications of antibiotic resistance, is essential to improve public health responses and manage disease outbreaks effectively.