Bacteria and Viruses

Bacteria
Bacteria are microscopic, unicellular prokaryotic organisms. They are the most abundant life forms on Earth, found in soil, water, air, and inside the bodies of larger organisms.

Structure:
No membrane-bound nucleus
Cell wall made of peptidoglycan
May have flagella (tail-like structures for movement)
May have pili (hair-like structures for attachment)
Some have a capsule (outer protective layer)
Ribosomes present for protein synthesis

Shapes of Bacteria:
Cocci = spherical (e.g., Staphylococcus)
Bacilli = rod-shaped (e.g., Escherichia coli)
Spirilla = spiral-shaped (e.g., Helicobacter pylori)

Useful Bacteria:
Lactobacillus: converts milk into yogurt
Rhizobium: fixes nitrogen in soil, benefiting plants
Used in production of antibiotics, vitamins, and probiotics

Harmful Bacteria:
Mycobacterium tuberculosis: causes Tuberculosis (TB)
Salmonella typhi: causes Typhoid fever
Vibrio cholerae: causes Cholera

Viruses
A virus is a non-cellular entity consisting of a core of genetic material (either DNA or RNA) surrounded by a protein coat called the capsid. Some viruses also have an outer lipid envelope.

Viruses are not considered fully living because they cannot reproduce on their own, have no metabolism, and show no cellular structure. They are obligate intracellular parasites — they can only reproduce inside a living host cell.

How viruses cause disease:
The virus attaches to the surface of a host cell.
It injects its genetic material into the host cell.
The host cell's machinery is hijacked to produce thousands of copies of the virus.
New viruses burst out of the host cell (often destroying it) and infect other cells.

Examples of viral diseases:
Influenza (Flu) — caused by Influenza virus
Common Cold — caused by Rhinovirus
COVID-19 — caused by SARS-CoV-2
HIV/AIDS — caused by Human Immunodeficiency Virus
Hepatitis B — caused by Hepatitis B virus
Measles, Mumps, Rubella — all caused by different viruses
Dengue Fever — caused by Dengue virus (transmitted by mosquitoes)
Rabies — caused by Rabies virus (transmitted by animal bites)

Treatment:
Viruses cannot be treated with antibiotics (which only work on bacteria). They are treated with antiviral drugs and prevented with vaccines.