2.1.7 Viruses

Key Features of Viruses:

1. Structure:

• Viruses consist of nucleic acid (either DNA or RNA) enclosed within a protein coat called the capsid.
• Some viruses are surrounded by a lipid envelope, which helps them enter host cells.
• Viruses lack organelles, a cell membrane, or cytoplasm.

2. Size:

• Viruses are significantly smaller than bacteria, typically measuring 20–300 nm.

3. Replication:

• Viruses are obligate intracellular parasites, meaning they can only reproduce within a host cell.
• They inject their genetic material into the host, hijacking the host's machinery to replicate and assemble new viruses.

4. Dependency on Host:

• Unlike bacteria, which can survive independently, viruses rely entirely on a host cell for survival and reproduction.

Differences Between Viruses and Bacteria:

Feature	Bacteria	Viruses
Cell Type	Prokaryotic cells with cell membranes and organelles.	Non-cellular particles with no organelles or membranes.
Genetic Material	Circular strand of DNA.	Can be DNA or RNA, single or double-stranded.
Reproduction	Divide by binary fission.	Replicate only inside a host cell.
Size	Larger (1–5 μm).	Smaller (20–300 nm).
Independence	Can survive and multiply outside a host.	Require a host to survive.

Examples of Diseases Caused by Bacteria and Viruses:

1. Bacterial Disease: Tuberculosis (TB):

• Pathogen: Mycobacterium tuberculosis.
• Infection:
• Infects phagocytes in the lungs.
• Forms tubercles (sealed areas of infected phagocytes due to an inflammatory response).
• Symptoms:
• Symptomless initially as bacteria remain dormant in tubercles.
• When the immune system weakens, bacteria become active, causing:
• Breathing difficulties.
• Coughing.
• Weight loss.
• Fever.
• Outcome: TB can cause severe damage to lung tissue and may lead to death if untreated.

2. Viral Disease: HIV (Human Immunodeficiency Virus):

• Pathogen: HIV, which causes AIDS.
• Infection:
• Initially causes fever, tiredness, and headaches.
• HIV antibodies appear in the blood within weeks, indicating HIV positivity.
• Virus lies dormant until the immune system weakens, leading to AIDS.
• Symptoms of AIDS:
• Weight loss.
• Chronic diarrhoea.
• Dementia.
• Increased risk of cancers and opportunistic infections like TB.