Dear student 1).Increase growth of algae leads to decrease in oxygen in water bodies and this can be threat to aquatic life. 2).They form biofilms which can be a nuisance like plaque formed in teeth. 3).Diseases in plants like smuts and rusts which leads to high agricultural losses. 4).Some bacteria produce chemical compounds called toxins which are harmful like botulinum toxin. 5).Viruses can lead to activation of oncogenes leading to cancer. Regards