If only a few microbes enter the body, they will probably be overcome by the host's defenses. However, if a large number of microbes gain entry, the stage is probably set for disease. The likelihood of disease increases as the number of pathogens increases. The virulence of a microbe is often expressed as the ID50 (Infection Dose needed to infect 50% of a sample population). A highly virulent microbe could cause a disease with a smaller number of cells.

the backbone of the dna and rna compound is sugar phosphate.

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.