What causes diseases such as the cold flu chicken pox and hepatitis?

Viruses

Viruses are the cause of diseases such as the common cold, flu, chicken pox, and hepatitis. Viruses are tiny, infectious agents that can only replicate inside the living cells of an organism. They are not cells themselves, and they do not have a metabolism. Viruses are made up of a protein coat that encloses a core of genetic material, either DNA or RNA.

When a virus enters a cell, it uses the cell's machinery to make copies of itself. These copies are then released from the cell and can infect other cells. Some viruses can cause disease by damaging the cells they infect. Others can cause disease by disrupting the body's immune system.

Bacteria

Bacteria are another type of microorganism that can cause disease. Bacteria are single-celled organisms that have a cell wall, a cell membrane, and cytoplasm. Bacteria can live in a variety of environments, including soil, water, and the human body.

Some bacteria are harmful, while others are beneficial. Harmful bacteria can cause disease by producing toxins or by damaging the body's tissues. Some common bacterial diseases include pneumonia, strep throat, and tuberculosis.

Fungi

Fungi are a type of organism that includes yeasts, molds, and mushrooms. Fungi are not plants, and they are not animals. They have a cell wall, but they do not have a nucleus. Fungi can live in a variety of environments, including soil, water, and the human body.

Some fungi are harmful, while others are beneficial. Harmful fungi can cause disease by producing toxins or by damaging the body's tissues. Some common fungal diseases include athlete's foot, ringworm, and candidiasis.

Parasites

Parasites are organisms that live on or in another organism (the host) and get their food from that organism. Parasites can be animals, plants, or microorganisms.

Some parasites are harmful, while others are beneficial. Harmful parasites can cause disease by damaging the body's tissues or by stealing nutrients from the host. Some common parasitic diseases include malaria, schistosomiasis, and hookworm.