Viruses are not living cells — they are particles of genetic material (DNA or RNA) surrounded by a protein coat.
They cannot reproduce on their own — they must invade a HOST CELL and use the cell's own machinery to make copies of themselves.
This process destroys the host cell when new virus particles burst out.
Viruses cause disease by:
Destroying cells as they replicate.
Triggering an immune response (the fever, aches and fatigue you feel are largely your immune system fighting the virus).
Disrupting normal organ function.
Measles
CAUSE: Measles virus (a paramyxovirus).
TRANSMISSION: Airborne droplets — spread very easily through coughing, sneezing and close contact.
SYMPTOMS:
High fever.
Red, blotchy skin rash spreading from face to body.
Runny nose, cough, sore red eyes.
White spots inside the mouth (Koplik's spots — a diagnostic sign).
COMPLICATIONS: Measles can be very serious — can lead to pneumonia (lung infection), encephalitis (brain inflammation) and death, particularly in malnourished children or immunocompromised individuals.
PREVENTION: The MMR vaccine (measles, mumps, rubella) — given in two doses in childhood. Highly effective.
TREATMENT: No specific antiviral drug — rest, fluids, paracetamol for fever. The immune system clears the infection.
HIV and AIDS
CAUSE: HIV (Human Immunodeficiency Virus) — a retrovirus.
TRANSMISSION: Through bodily fluids — unprotected sexual contact, sharing needles, blood transfusions (in countries without screening), mother to baby (during pregnancy, birth or breastfeeding).
WHAT HIV DOES: HIV specifically targets and destroys CD4+ T-helper lymphocytes — the very cells that coordinate the immune response. Over time, the immune system is progressively weakened.
AIDS: AIDS (Acquired Immune Deficiency Syndrome) develops when HIV has destroyed so many lymphocytes that the immune system can no longer function. The patient becomes vulnerable to OPPORTUNISTIC INFECTIONS — infections that a healthy immune system would easily fight off (e.g. rare pneumonias, certain cancers).
PREVENTION: Condoms, clean needles, screening blood supplies, antiretroviral drugs for pregnant women with HIV.
TREATMENT: ANTIRETROVIRAL DRUGS (ARVs) — prevent HIV from replicating. Cannot cure HIV but allow patients to live near-normal lives. Must be taken for life.
Tobacco Mosaic Virus (TMV)
CAUSE: Tobacco mosaic virus — affects a wide range of plants including tobacco, tomatoes, peppers and cucumbers.
TRANSMISSION: Direct contact between plants, contaminated tools, insects feeding on plants, and handling by humans.
SYMPTOMS: A distinctive MOSAIC PATTERN of light and dark green or yellow patches on leaves. Leaves may also be distorted or stunted.
WHY IT'S HARMFUL: Infected cells cannot produce chlorophyll properly. The mosaic-patterned leaves have LESS CHLOROPHYLL than healthy leaves → reduced rate of photosynthesis → plant grows poorly and produces less fruit/yield.
PREVENTION: Removing and destroying infected plants, cleaning tools, using virus-free seeds, controlling insect vectors.
⚠️ Common Mistake
HIV and AIDS are NOT the same thing. HIV is the VIRUS. AIDS is the CONDITION that develops when HIV has destroyed enough lymphocytes that the immune system collapses. You can have HIV for many years without having AIDS if treated with ARVs. Also: antibiotics DO NOT treat viral infections — not measles, not HIV, not flu.