In 1346 – 1353 around 200 million people died of a deadly plague known as Black death. The plague was so severe that it approximately wiped out 50% of total european population.
The virus originated from rat flea carried by rats and spread through ships that used to ferry for trade.
Recently, we have seen the havoc Ebola virus created. Ebola is RNA based virus. It means it has genetic material that contains instructions to create new virus particles. Once the virus finds its way inside the cell membrane, it replicates itself and compromises critical organs like liver, immune system, blood vessel cells. Ultimately host (human) dies of internal bleeding or haemorrhage.
Studies suggest that Ebola was carried by bats which again is notoriously known for carrying virus and parasites.
With constant evolution in medical field, new antibiotics are engineered to eliminate viruses. However, due to natural selection virus also mutate and become more competant.
HIV virus is a perfect example of this. When HIV virus penetrates into the body. It starts multiplying. The immune system starts to fight back but virus has a upper hand in the fight. In order to combat the virus, antibiotics that are determined to kill the virus are injected.
Positive results start showing, antibiotics start eliminating the virus from the body. However, the virus mutates and learns to stay immune to the antibiotics. Virus changes strategy with every mutation and the body is never able to detect or combat. Eventually patient dies.
Its almost paradoxical that the process that helped in our creation can kill us. We can only hope that we keep outpacing virus in this never ending battle but keep in mind 1 contagious sophisticated virus can annihilate and erase entire humankind with all its nuclear bombs and arsenal.