Infectious diseases, at a glance

An infectious disease is caused by a transmissible pathogen. The pathogen, disease and transmission route are different parts of the explanation.

• Pathogen = cause; disease = condition; route = how the pathogen travels.
• Infectious diseases can be transmitted; non-infectious diseases cannot.
• Use the specific route given, not a single route for every pathogen.

Keep in mind: Family members can share genes and environments as well as exposure to pathogens. Similar illness alone does not establish person-to-person transmission.

A bacterium is a cell with DNA not enclosed in a nucleus. A virus has genetic material inside a protein coat and replicates only in living host cells.

• Bacterium: cell wall, membrane, cytoplasm, ribosomes, DNA without a nucleus.
• Virus: genetic material enclosed by a protein coat.
• Viral replication requires a living host cell.

Keep in mind: Typical bacteria themselves have no membrane-bound nucleus. Viruses have a different organisation: genetic material inside a protein coat and dependence on living host cells for replication.

Influenza is caused by influenza viruses; pneumococcal disease is caused by pneumococcus bacteria. Their symptoms can overlap.

• Influenza -> influenza virus.
• Pneumococcal disease -> pneumococcus bacterium.
• Similar symptoms do not prove the same pathogen.

Keep in mind: Pneumococcus can cause pneumonia, but other bacteria and viruses can also cause it. Pneumococcal disease also includes infections at other sites.

A useful prevention explanation names the transmission route and shows exactly which part of it is interrupted.

• Respiratory particles and secretions can transfer respiratory pathogens.
• A person may carry pneumococcus without obvious illness.
• Explain the link between the action and the route it interrupts.

Keep in mind: Some infected people or carriers have no obvious symptoms. Pneumococcus can be carried in the nose or throat without causing illness.

Vaccination exposes the immune system to an agent resembling a pathogen, forming immune memory that enables a faster antibody response on later exposure.

• Vaccination stimulates the body's own immune response.
• Antibodies are specific to matching antigens.
• Memory cells enable a faster response on later exposure.

Keep in mind: Vaccination stimulates the body's immune response and memory. Antibiotic action and the transfer of ready-made antibodies are different mechanisms.

Antibiotics act on susceptible bacteria. Exposure can select resistant bacteria, which survive and reproduce; the bacteria become resistant, not the person.

• Bacterial targets are absent from viruses.
• Selection: variation -> exposure -> differential survival -> reproduction.
• Resistant bacteria, not an antibiotic-resistant person.

Keep in mind: Resistance is a property of bacteria to particular antibiotics. Antibiotic use selects bacteria; it does not make the person an antibiotic-resistant organism.