How come every person who comes in contact with a diseased individual not get infected?

Not every person who comes in contact with a diseased individual gets infected for several reasons:

1. Immune System: A person's immune system plays a crucial role in defending against infections. It consists of various components, such as white blood cells, antibodies, and cytokines, that work together to recognize and eliminate pathogens. A robust immune system can effectively fight off infections, reducing the likelihood of developing a disease.

2. Exposure: The extent of exposure to the disease-causing agent (pathogen) influences the risk of infection. Factors like the duration and intensity of contact with the pathogen, as well as the mode of transmission (e.g., airborne, direct contact, or through bodily fluids), all play a role in determining the likelihood of infection.

3. Initial Dose: The initial dose of the pathogen matters. Some diseases require a certain threshold of pathogen concentration to establish an infection. If the dose of the pathogen is low or insufficient, the immune system may be able to overcome it without becoming overwhelmed, preventing the onset of the disease.

4. Host Factors: Individual variations in genetic makeup, underlying health conditions, age, and nutritional status can impact susceptibility to infections. Certain genetic traits or chronic conditions may compromise immune function, making individuals more vulnerable to infections.

5. Pathogen Factors: The characteristics of the pathogen itself, such as its virulence (ability to cause disease) and infectivity (ability to spread), influence the likelihood of infection. Some pathogens are inherently more virulent or have evolved mechanisms to evade the immune system, making them more successful at causing disease.

6. Environmental Factors: Environmental conditions can also affect infection risk. Factors like temperature, humidity, and sanitation practices play a role in the survival and transmission of pathogens.

7. Vaccination and Prior Exposure: Prior exposure to a pathogen through vaccination or natural infection can induce immunological memory, providing protection against future infections from the same pathogen. This is the principle behind vaccination programs, which aim to stimulate immunity without causing severe illness.

Therefore, various factors related to the individual's immune status, the nature of the pathogen, exposure conditions, and environmental factors collectively influence whether a person develops an infection after coming into contact with a diseased individual.