Infectious diseases

Infectious diseases are a central topic in biology, defined as illnesses or disorders of the body or mind that lead to poor health and are caused by pathogens that can be transmitted from person to person. They are often referred to as communicable or contagious diseases.

1. Definition and Distinction

• A disease is broadly defined as an unhealthy condition of the body, which reduces the effectiveness of an organism's function.

• Infectious diseases are specifically those caused by invading organisms living parasitically on or in the body. They are transmissible.

• Non-infectious diseases, conversely, are not "caught" from other organisms and include conditions like lung cancer, sickle cell anaemia, malnutrition, and inherited disorders.

2. Causative Agents (Pathogens) Pathogens are biological agents that cause disease. They can be various types of microorganisms:

• Viruses: Acellular (not cells), non-living structures composed of nucleic acids (DNA or RNA) surrounded by a protein coat called a capsid. They must invade and reproduce inside host cells, using the host's "machinery" for replication. Examples include HIV, influenza, measles, and smallpox.

• Bacteria: Single-celled, prokaryotic microorganisms. They can produce poisonous waste products called toxins that damage host cells and upset body systems, causing symptoms like fever, headache, and diarrhoea. Examples include Vibrio cholerae (cholera), Mycobacterium tuberculosis (TB), and Clostridium tetani (tetanus).

• Protoctists: Eukaryotic organisms. Plasmodium, which causes malaria, is an example.

• Fungi: Some fungi are pathogenic, causing diseases like athlete's foot.

3. Transmission of Pathogens Pathogens can spread through various means, forming a "transmission cycle" that control methods aim to break:

• Direct Contact: Through intimate human contact, such as sexual intercourse (e.g., HIV), or sharing hypodermic needles. Smallpox was also transmitted by direct contact.

• Airborne Droplets (Inhalation): Coughs or sneezes release tiny droplets containing bacteria or viruses that can be inhaled. Examples include TB, measles, and influenza.

• Water and Food: Contaminated water or food can transmit bacteria. Cholera, for example, is primarily water-borne and food-borne, spreading through faecal contamination. Mycobacterium bovis (a cause of TB) can be spread via undercooked meat and unpasteurised milk.

• Vectors: Organisms that transfer a disease or parasite from one host to another. Female Anopheles mosquitoes are vectors for malaria, transmitting Plasmodium when they take a blood meal.

4. Prevention and Control Controlling infectious diseases requires understanding the pathogen's biology and transmission.

• Sanitation and Hygiene: Providing clean water, treating sewage, and maintaining good personal hygiene (e.g., handwashing) are crucial for diseases like cholera.

• Vector Control: Reducing vector populations, such as mosquitoes, through insecticide spraying or draining breeding sites, is vital for diseases like malaria.

• Vaccination: Vaccines contain antigens that stimulate the body to produce memory cells against a pathogen, conferring immunity without causing the disease. This builds "herd immunity," protecting individuals and populations by reducing disease occurrence.

• Drug Therapy:

  • Antibiotics are used to treat bacterial infections by killing or inhibiting bacterial metabolism. They are effective because they target specific components of prokaryotic cells (like cell wall synthesis or ribosomes) that are absent or different in human cells.

  • Antibiotics are ineffective against viruses because viruses use the host cell's enzymes and ribosomes for replication, so they lack specific viral targets for antibiotics.

  • Antivirals: Used for viral infections like HIV, they aim to slow down viral replication.

• Early Diagnosis and Treatment: Prompt diagnosis and treatment, like oral rehydration therapy for cholera or anti-malarial drugs, significantly improve outcomes and reduce spread.

• Isolation: Isolating infected individuals, especially for highly contagious diseases like TB, helps prevent spread.

5. Specific Infectious Diseases and Their Characteristics

• Cholera:

  • Pathogen: Bacterium Vibrio cholerae.

  • Transmission: Water-borne and food-borne, through faecal contamination.

  • Symptoms: Severe diarrhoea, dehydration, and loss of salts.

  • Control: Clean water supply, effective sewage treatment, and oral rehydration therapy. Vaccines exist but have limited effectiveness.

• Malaria:

  • Pathogen: Protoctist Plasmodium (several species, P. falciparum being most dangerous).

  • Transmission: Female Anopheles mosquitoes (vector). Also possible via blood transfusion and unsterile needles, or mother to child across placenta.

  • Symptoms: Cyclic high temperatures, headache, nausea, shivering, enlarged spleen, anaemia.

  • Control: Reducing mosquito numbers, avoiding bites (nets, repellents), and anti-malarial drugs. Difficult to develop a vaccine due to complex life cycle and antigenic variation.

• Tuberculosis (TB):

  • Pathogen: Bacterium Mycobacterium tuberculosis (and M. bovis).

  • Transmission: Airborne droplets from coughs/sneezes, especially in crowded conditions. M. bovis from contaminated meat/milk.

  • Symptoms: Racking cough, coughing blood, chest pain, shortness of breath, fever, sweating, weight loss.

  • Control: Antibiotic treatment (often prolonged and multi-drug), BCG vaccine (variable effectiveness), and addressing poor living conditions. High death rate in HIV-positive individuals due to weakened immune system.

• HIV/AIDS:

  • Pathogen: Human Immunodeficiency Virus (HIV), a retrovirus containing RNA.

  • Transmission: Exchange of bodily fluids (sexual intercourse, shared needles, blood transfusions, mother to child via placenta or breast milk). No vector.

  • Impact: Infects and destroys T-helper cells (CD4 cells), leading to acquired immune deficiency syndrome (AIDS). AIDS patients become vulnerable to opportunistic infections (e.g., pneumonia, TB, cancers).

  • Control: Antiretroviral drugs to slow replication, education, safe sex practices, needle exchange, blood screening. No vaccine or cure yet, partly due to antigenic variation.

• Measles:

  • Pathogen: Virus (Morbillivirus).

  • Transmission: Airborne droplets, highly infectious.

  • Symptoms: Fever, rash, flu-like symptoms. Can lead to serious complications.

  • Control: Vaccination is the best defense. Babies can gain natural passive immunity from their mothers via the placenta. Eradication is difficult due to population dynamics and varying vaccine response.

• Smallpox:

  • Pathogen: Variola virus (DNA virus).

  • Transmission: Direct contact, inhalation of droplets.

  • Status: Successfully eradicated worldwide by 1980. Reasons for success include a stable virus (no antigenic variation), direct transmission (allowing ring vaccination), and an effective freeze-dried vaccine.

6. Global Impact and Challenges Infectious diseases remain a major global health problem, particularly in developing countries due to factors like poverty, overcrowding, poor sanitation, and limited access to healthcare. Global travel can facilitate rapid spread of infections across continents. The "arms race" between pathogens and hosts is ongoing, as pathogens evolve resistance (e.g., antibiotic resistance) and antigenic variation.