Lesson · B.5.1

Pathogens and infectious disease

The HESI A2 Biology subtest asks you to sort pathogens by type and trace how an infection moves from reservoir to host. Here is the chain of infection and the immune defences that break it, in plain terms.

ObjectiveB.5.1Read5 minExam weightMicrobiology and Immunity ≈ 16% of Biology

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What you will be able to do

Classify a pathogen as bacterial, viral, fungal, or protozoan from a short description.
Name the six links in the chain of infection and give one way to break each link.
Distinguish the innate immune response from the adaptive immune response, including the roles of neutrophils, B cells, and T cells.

What the lesson concludes

Gram staining sorts bacteria into Gram-positive and Gram-negative, and that difference shapes which antibiotics will work.
Viruses need a host cell to replicate; bacteria, fungi, and protozoa do not.
The chain of infection has six links, and breaking any one of them stops transmission.
Innate immunity acts fast and non-specifically; adaptive immunity is slower but remembers the pathogen.
Memory B cells and T cells are why a second infection with the same pathogen is usually milder or absent.