The name Typhus comes from the Greek word ‘typhos’ meaning cloudy. It describes the state of mind of the infected people. Rickettsia is the causative organism, which is a strict parasitic bacteria. They are unable to survive outside the living cells.
This group of diseases consists of;
1. Epidemic typhus
2. Recrudescent typhus (Brill- Zinsser disease)
3. Endemic typhus
Epidemic Typhus
Louse borne typhus, classical typhus and Goal typhus are the commonly referred other names. It had been occurring in devastating epidemics during the times of war and famine.
The causative agent of epidemic typhus is R. prowazekii. It is named after von Prowazek, who died of typhus fever while investigating the disease. Humans are the only natural vertebrate hosts.
Several animals- guinea pigs, mice, cotton rats and gerbils (desert rat) - may be infected experimentally. Natural infection in flying squirrels had been reported. They may possibly act as reservoir hosts, infection being spread by the squirrel louse and flea.
 The human body louse Pediculus humanus corporis is the vector. The head louse may also transmit the infection, but not the pubic louse (parasitic insects found in the genital area). The lice become infected by feeding on patients.
The Ricettsiae multiply in the gut of the lice and appear in the feces in 3-5 days. Lice give way to the infection within 2- 4 weeks, remaining infective till they die. They can transmit the infection after about a week of being infected.
Lice may be transmitted from one person to another. They are highly sensitive to temperature, so are in constant look out for new hosts from feverish patients. Lice defecate while feeding.
Infection may be transmitted when the contaminated louse feces are rubbed through minute abrasions caused by scratching. At times, the infection may also be transmitted by aerosols of dried louse feces through inhalation or through the conjunctiva.
The incubation period is 5- 15 days. The fever starts off with fever and chills. A characteristic rash appears on the 4th or 5th day. It first appears on the trunk, then spreads to the limbs, palms and soles. It spares the face.
The patient develops delirium towards the second week.  Some of the infection in certain patients remains latent which may be reactivated. This is may lead to recrudescent typhus (Brill- Zinsser disease).
Brill- Zinsser disease
it explains the manner in which the Rickettsia is able to survive without extra human reservoirs.
Endemic typhus
Flea- borne typhus is a milder disease. It is caused by R. typhi. Which is maintained in nature as a mild infection of rats, transmitted by rat fleas; Xenopsylla cheopis.The bacteria multiplies in the gut of the flea and is shed through the feces. The flea is unaffected, but remains infectious for the rest of its natural span of life. Human infection is the dead end.Humans acquire the disease usually through the bite of infected fleas, where their saliva or feces is rubbed in or through aerosols of dried feces. Ingestion of food recently contaminated with rat urine or rat flea may also cause the infection.
R. typhi & R. prowazekii are closely related and so they differentiated by the characteristic Neill-Moose or tunica reaction.