Cambridge O Level Biology · Syllabus 5090 · Disease and Immunity
Pathogen
What is Pathogen?
A disease-causing organism. Bacteria, viruses, fungi and parasites can all be pathogens; the term describes the role an organism plays in causing disease, not the group it belongs to, and it does not include an organism that merely carries a pathogen from one host to another.
This definition is part of the Disease and Immunity chapter in Cambridge O Level Biology.
Pathogen in context
A pathogen is a disease-causing organism, and a transmissible disease is one in which the pathogen can pass from one host to another. Every method of disease control in this chapter works by breaking one specific link in the chain that runs from an infected host, along a route of transmission, past the body's barriers, into a new host. Vaccination is the one control that acts on the far end of the chain: it prepares the immune system before the pathogen ever arrives.
Common mistakes with Pathogen
- "Every disease is transmissible." Transmissible means a pathogen can pass from one host to another. Diseases caused by diet, by inherited alleles or by the body's own processes have no pathogen to pass. Write A transmissible disease is one in which the pathogen can be passed from one host to another; many diseases are not transmissible.
- "The mosquito causes malaria." The mosquito is the vector. A parasite is the pathogen. Naming the wrong organism as the cause loses the mark even when the rest of the transmission chain is correct. Write Malaria is caused by a parasite, which is transmitted from person to person by a mosquito acting as a vector.
- "Antibodies engulf pathogens." Antibodies are proteins. A protein cannot surround and swallow anything. Phagocytes are cells, and they engulf. Write Antibodies bind to antigens, which can clump pathogens together or mark them so that phagocytes engulf and digest them.
- "Phagocytes produce specific antibodies." Phagocytes are non-specific and do not produce antibodies. Lymphocytes do. Write Lymphocytes produce specific antibodies and form memory cells; phagocytes engulf and digest pathogens.
- "Antigens are made by lymphocytes." Antigens belong to the pathogen. Lymphocytes make antibodies against them. Write An antigen is a molecule, usually on the pathogen's surface, that stimulates a specific immune response.
- "Vaccines always contain live pathogens." They may contain weakened pathogens, inactivated pathogens, or the antigens alone. Write A vaccine contains antigens from the pathogen, in a form that does not cause the disease.
- "Skin and mucus stop all pathogens getting in." If they did, nobody would ever be ill. Barriers reduce entry; they do not prevent it. Write First-line barriers reduce the number of pathogens that enter the body, but they do not prevent all infection.
Questions students ask about Pathogen
Is a virus a living organism, and does that matter for the definition of a pathogen?
For O Level purposes, treat a virus as a pathogen. The syllabus definition of a pathogen is "a disease-causing organism", and viruses are named as pathogens throughout the topic. The debate about whether viruses are alive is not examined; what is examined is that a virus is genetic material in a protein coat that can only reproduce inside a host cell, which is why antibiotics do not affect it.
If a vaccine contains the pathogen, why does it not give you the disease?
Because of the form it is in. A vaccine may contain weakened pathogens, inactivated pathogens, or just the antigens, and none of these can establish a normal infection. What survives is the antigen's shape, which is all the lymphocytes need in order to produce complementary antibodies and form memory cells.
Why do some vaccines need booster doses if active immunity is long lasting?
Because "usually long lasting" is not the same as permanent. For some pathogens the memory-cell response weakens over years, so a further dose is given to stimulate the response again. This is still active immunity: the person's own lymphocytes do the work each time.