Pathogens include all but which one of the following? disease-causing bacteria, helpful bacteria found in the gut, viruses, microscopic parasites

Helpful bacteria found in the gut are not pathogens.

Explanation:

An organism is said to be pathogenic if it can invade, grow and multiply in an individual host’s body and cause disease in the susceptible host’s body by evading the immune system of the host.

Viruses belong to obligate pathogen because they require the cellular machinery of the host’s body to grow and replicate and thereby will cause disease in the host.  

Disease-causing bacteria are pathogenic like Salmonella, C. difficile, etc.  

Helpful bacteria of the gut also called the human gastroinstestinal or gut microbiota, or the gut flora are those bacteria living in the digestive system of the body and improve the gut health. Gut bacteria like Lactobacillus generally help by:

1) gene flow

- it is described as the transfer of genes from one population to another population so that frequency of gene is increased.

-a zebra migrates to join a different herd of zebras.

2) natural selection

- a mechanism of evolutionary change in which individuals with advantageous traits survive and reproduce whereas the others ( those that can not adapt) eliminate from the population.

-competition for sunlight leads to taller trees.

3) mutation

- permanent change in the nucleotide sequence of dna ( genetic material) that could have harmful or useful consequences.

-the dna of a snake changes to make its venom stronger.

4) genetic drift - it is a random phenomenon ( that occurs by chance such as fire) that results in change in the gene frequency in a small population.    

-a grassfire randomly sweeps through a population of buffalo and kills most of the animals.

explanation: