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What are some human diseases caused by virus and what are their respective modes of transmission?

The main viral diseases transmitted by respiratory secretions (sneezes, cough) and by saliva drops are flu, mumps, smallpox (variola, already considered eradicated), rubella, measles, SARS. Main viral diseases transmitted through blood or sexual contact are AIDS, hepatitis B, hepatitis C, HPV, ebola hemorrhagic fever. Main viral diseases transmitted by animal vectors are rabies, dengue fever, yellow fever. Some viral diseases transmitted by fecal-oral route, including contaminated food, are hepatitis A, poliomyelitis (disease almost eradicated in many parts of the world).

What is the virus that causes flu? Why does not the body produce permanent immunity against that virus? How does the vaccine against flu work?

Flu is a disease caused by the influenza virus, a highly mutant DNA virus. Due to the high mutation rate of the virus, that forms many different strains, flu always present epidemic features in affected populations and people may have several flu episodes during life (the immune response made from previous infections is not efficient in future infections).

The vaccine against flu is a vaccine made of attenuated virus of three different strains. Each year the WHO (World Health Organization) researches and determines which are the strains that should compose the vaccine. This is a strategy to face the high mutation rate of the virus.

Why is rubella during gestation a threat to the fetus?

If occurring during gestation rubella is a dangerous disease because the virus crosses the placenta and contaminates the fetus. The fetus then develops congenital rubella, a teratogenic (cause of malformations) disease.

Congenital rubella may be prevented by vaccination. Doctor must always be consulted before vaccination.